Xerneas has two forms: Neutral Mode and Active Mode. There is no gameplay difference between them; Xerneas appears in Active Mode in battle, and appears in Neutral Mode in situations outside of battle, such as cutscenes, the Pokédex, the Pokémon screen, and Pokémon-Amie. According to the Pokédex, Xerneas shares everlasting life while it is in Active Mode.
There's no real noticeable difference between the two forms, other than aesthetic; one is its battle-ready form, and the other is its passive form.
Source
Hope this helped!
