Hi, Khosro, "Welcome home!" is correct because "home" is an adverb of place in this sentence. It shows where the action occurs. Because you can't use a preposition with an adverb alone, "to home" is not possible. "Welcome to home!" is never correct because "home" is a singular, count noun and requires a determiner, such as my, her, our, John's: Welcome to our home! You could use "the," but then additional information is necessary to define "home": Welcome to the home of the best pizza on the planet!

welcome home, is spoken to a person who belongs in the home. welcome to home, does not really exist welcome to my home, is spoken to a visitor to the speaker's home welcome to your home, is spoken to a person coming into his home for the first time or after a long period away