Here is a good summary of K-1 Fiance Visas:
Overview: What Is a K-1 Fiance Visa?
The fiancé(e) K-1 nonimmigrant visa is for the foreign-citizen fiancé(e) of a United States (U.S.) citizen. The K-1 visa permits the foreign-citizen fiancé(e) to travel to the United States and marry his or her U.S. citizen sponsor within 90 days of arrival. The foreign-citizen will then apply for adjustment of status to a permanent resident (LPR) with the Department of Homeland Security (DHS), U.S. Citizenship and Immigration Services (USCIS). Because a fiancé(e) visa permits the holder to immigrate to the U.S. and marry a U.S. citizen shortly after arrival in the United States, the fiancé(e) must meet some of the requirements of an immigrant visa. Eligible children of K-1 visa applicants receive K-2 visas.
Under U.S. immigration law, a foreign-citizen fiancé(e) of a U.S. citizen is the recipient of an approved Petition for Alien Fiancé(e), Form I-129F, who has been issued a nonimmigrant K-1 visa for travel to the United States in order to marry his or her U.S. citizen fiancé(e). Both the U.S. citizen and the K-1 visa applicant must have been legally free to marry at the time the petition was filed and must have remained so thereafter. The marriage must be legally possible according to laws of the U.S. state in which the marriage will take place.
In general, the foreign-citizen fiancé(e) and U.S. citizen sponsor must have met in person within the past two years. USCIS may grant an exception to this requirement, based on extreme hardship for the U.S. citizen sponsor to personally meet the foreign-citizen fiancé(e), or, for example, if it is contrary in the U.S. citizen sponsor’s or foreign-citizen fiancé(e)’s culture for a man and woman to meet before marriage.