Visiting Mexico simply requires a valid Passport and completion of a simple tourist card (FMT…Forma Migracion Turista) which you will be provided by your airline prior to landing in Puerto Vallarta.  The FMT is valid for up to 180 days and is required to be surrendered to immigration authorities when you leave Mexico.
If you wish to stay longer than 180 days at a time, you will be required to apply for an FM3 (Forma Migracion Tres) that enables you to live and work in Mexico legally on a long-term basis.  A FM3 is required for those wishing to stay in Mexico for longer than 180 days, those looking to work or start a business in Mexico, or foreigners who desire to have a Mexico driver’s license.