A K-1 visa is a visa issued to the fiancé or fiancée of a United States citizen to enter the United States. A K-1 visa requires a foreigner to marry his or her U.S. citizen petitioner within 90 days of entry, or depart the United States. Once the couple marries, the foreign citizen can adjust status to become a lawful permanent resident of the United States (Green Card holder).
The K-3 visa is for the foreign spouse of a U.S. citizen. It was designed to shorten the physical separation between the foreign national and his or her U.S. citizen spouse. Specifically, it allows the foreign national to enter the United States to await approval of the immigrant visa petition. The K-3 Visa is for Spouse of a United States Citizen. The purpose of the K-3 visa is to promote family reunion and serves as a temporary remedy for the long delayed family based immigration petition process. It allows the alien spouse to stay in the U.S. while waiting for the result of the pending immigration petition.