Mew is a Psychic-type, Very Rare Pokémon. It does not evolve into or from any Pokémon, and has no Novelty forms. It can be obtained via the Forgotten Memories Exploration and, like all other Very Rare Pokémon, extremely rarely in the Lab.
Mew is known particularly for being owned by Professor Cypress, a bot that can breed her Mew with Ditto to produce any Pokémon available on GPX+. This includes all Very Rare Pokémon, special Pokémon such as Ditto and Unown, and any Novelty Pokémon.