The last name Kim is the most common Korean surname. It makes up 22 percent of the Korean population, which is equivalent to almost 10 million people. In contrast, there are rare family names that number less than 1,000, such as Pung or Sop. For over 1,000 years, Koreans have maintained the unique system of using their regional bases as an important way to differentiate their identities. So most Kims almost certainly do not share the same bloodline. There are 285 regional origins for the Kims, such as the Kyongju Kim clan and Kimhae Kim clan. Consequently, in order to find out the origin of a particular Kim, one would have to identify which clan they are descended from and research that particular clan. Ok, now the long answer: Most commoners in Korea did not begin to adopt surnames until the late 10th century. During the Three Kingdoms period, which lasted until the late seventh century, using surnames was a privilege given to a limited number of social groups such as royal clans and the aristocracy. Each of the ancient three kingdoms had, at most, 10 surnames such as Ko for the royal clans for Koguryo and Paekche, and Park, Sok and Kim for the three different royal clans of Silla. King Taejo, launching the Koryo Dynasty in the early 10th century, bestowed his loyal followers with last names and distinguished their surnames by their regional bases, giving birth to the modern "ponkwan" system. Ponkwan refers to the regional distinction of surnames. He also condemned a band of rebellious strongmen in the present-day Mokchon area of South Chungchong Province by giving them "animal" surnames of pig, horse, elephant and cow, and graced other loyal subjects by allowing them to share his own surname, Wang. During the Choson Dynasty (1392-1910), most commoners, except for slaves and outcasts, had surnames, as registered in period tax documents and conscription lists. Listed in an annex to the official history of King Sejong in the mid-15th century are 265 surnames, while those discussed in a late 18th century document on lifestyle total 298, including an array of defunct, archaic surnames of that time. Toward the end of the dynasty, a growing number of affluent commoners and entrepreneurs, who belonged to the lower echelons of the social hierarchy, secretly bought the status of nobility and adopted "dignitary" yangban surnames. Adopted under the influence of the Japanese imperialists who sought to control Korea more effectively, the monumental decree required all Korean nationals to register their full names and regional origins. At that time, those without surnames often adopted the surnames of their masters or were simply assigned one by recording officials. Compared to China and Japan, Korea has a far smaller number of surnames, among which the "big four" surnames - Kim, Lee, Park and Choi - dominate the scene. The extremely populous China hosts countless surnames, and Japan is home to some 132,000 surnames, a figure that is beyond comparison with Korea, even when one takes into account that its population is twice that of Korea. The big four surnames were adopted by commoners in the course of Korean history since these surnames were simply perceived to be of blue blood. These names were also sometimes bestowed upon loyal servants by royalty or nobility. The Kim and Park clans were royal clans of the ancient Silla Kingdom.

Graybyun's answer is great. (Where did you get the infos?) But I think that it is rather an explanation for the general Korean surnames, not for the origin of the "Kim" specifically. As a member of Korean "Kim" family, I want to answer to this question. There are two major Kims in Korea, 'Kyongju' Kim and 'Kimhae' Kim. I am a descendant of 'Kimhae' Kim. So, here I'll just discuss 'Kimhae' Kim only. I hope that someone who has "Kyongju" Kim will explain their origin. The founder of Kimhae Kim was King Suro, who was born from a golden egg. There is a place called Gujibong (small mountain) in Kimhae. One day in year 42 AD, there came six golden eggs, and the next day they all turned to six men. King Suro came out of the egg the first, and he was given the name "Kim", which means "gold" in Chinese Character. (Chinese Character is not only for Chinese. It is used in all Asia.) King Suro soon acceded to the throne and married to a princess named Hwang-ok Huh from Ayuta country. I know this story is a myth. But it represents the antient 6 Gaya nations and it is very important story in Korean history. This story was told and handed downd from generation to generation. I am the seventieth generation of King Suro. (Samyeon clan - for those who want to know deeply)

In addition to everything said above. My last name is also Kim of the KimHae Kim clan. Many people over the years have adopted the KimHae Kim and Kyongju Kim last names (which are all the same character 'Kim' just with different clans) because they were considered "blue blood" and enjoyed power as Royal Families of Kaya and Shilla Dynasties and as Noble Families (in the strict neo-confucian caste system during Koryo and Yi Dynasties) until the end of the Yi Dynasty (1910). Therefore, not every "Kim" has a Chokbo (Family Genealogy) that proves that they are truly a "Kim" of Kimhae or Kyongju clans. Unfortunately, the Korean culture to distinguish people into social castes still exists today in that when you meet someone, they always ask you, "what is your last name?" and "What clan?" I hope it'll go away soon - it's a waste.

"Kyongju" Kim Origin goes 4th King of Shilla (King Tal-Hae) was walking through the woods, and suddenly sky turned purple and Rooster was crying. So he followed the sound and there he saw a golden chest and inside the chest there was a boy. So the King Tal-hae named him Kim Alji which means golden egg. Roosters were considered sacred in Shilla as well as Gaya. It represented birth and renewal. Kim Alji is the ancestor of all Shilla dynasty Kims. Kim Alji's 7th generation became first Kim to be Kinged in Shilla, King Michoo he was the 13th King of Shilla. Of the 56 Kings of Shilla 38 of them were Kims. King Kyong-Soon was the last King of Shilla and 28th generation form Kim Alji. Kingdom became very corrupt and after many rebellions, King Kyong-Soon surrendered his crumbling kingdom to Koryo. There he received his family name Kyongju Kim. Kyongju was a capital of Shilla. It was called surabul before Koryo, but Koryo dynasty changed the name of the city to Kyongju.To prevent Shilla nationalism which would cause rebellion. There are 623 different variations of recorded Shilla Kims and 100's not verified. ( different clans and also whose last name that has been changed but share same ancestry) Many people especially during war times received last name or clan name, when they had exceptional merits. Although Kim generally translated as a Gold, it also means steel. At about 7 or 5 bce(not sure about the exact date) southern part of Korea (Gaya and Shilla not a country yet) received technology from Chinese to manufacture steel after 100 yrs later southern Korea became #1 exporter and maker of steel to both Chinese and Japan. Makers of steel became very rich and powerful, enough to form a country. Gaya was the first major country to form in southern Korea. It was a very powerful nation with full steel armor and barding on horses. Since Gaya was known for their steel this maybe why they named their last name to Kim. While Shilla was known for their gold, but there is a theory that the Kim Alji actually may have came from Gaya. Because Kimhae Kim does have older history than the Kyongju Kim, Kim Alji may have passed down metal working to Shilla and became a royal family instantly.

people might want to disagree with me and get mad.... but a lot of nameless people adopted the last name, including lower class people so not all kimhae kims are actual blood descendents or royalty. they blended in and becuz of this, kimhae kims ares the most kims today. ofcourse you cant always find out whos real or adopted kimhaekim but... real kimhaekims have a certain birthmark. for guys, its a birthmark on their genital.