Visa K-1 or Visas for Fiancé(e)s of U.S. Citizens:
The K-1 visa is intended for the spouses of US citizens who wish to enter the United States to marry.
The fiancé(e) will be entitled to the K-1 visa if:
Although it is possible to enter the United States with a tourist visa and then, as a result of marriage, request a green card, this practice should be discouraged. In fact, when you enter the US UU with a tourist visa, is declaring that you do not want to permanently stay in the United States; therefore, to use a tourist visa when in fact you have the intention to marry and immigrate permanently, corresponds to a false oath and can have serious consequences.
The K-3 visa is intended for spouses of US citizens who reside abroad and wish to enter the United States while the application for family reunification is approved. When this happens, in fact, the holder of the K-3 visa can request the Adjustment of Status to obtain the green card. To obtain the K-3 visa, you must pass a series of medical examinations and prove that the US citizen can guarantee the financial support of the spouse.