Xerneas is a pure Fairy-type Pokemon, so the Legendary is limited in options. It's Fairy-typing would be very useful in the Ultra League as long as it had the right moves to go along. However, normal-type moves are currently the best selection on Xerneas.
For the fast attack on Xerneas, players should use Zen Headbutt. It's a Psychic-type move with decent damage output. The other option is Tackle, which is a Normal-type attack. Both are very similar in damage per second and energy per second gaine. But the difference is in how effective the moves are, and Zen Headbutt will win more matchups.
When choosing the charge attack on Xerneas, players should prioritize Giga Impact. It has the highest damage output when the energy per second is taken into account. As the first option, it will be the most effective charge attack.
Players can choose a second charge attack though, and in that case, Moon Blast is a good second option. It won't damage as much as Giga Impact, but it will at least utilize the Fairy-typing of Xerneas.
Is Xerneas a useful option for PVP battles in Pokemon GO?
Xerneas is fresh in Pokemon GO, and many players may want to utilize the Pokemon, but they may want to wait.
Unfortunately, Xerneas is useless without a good Fairy-type move in Pokemon GO. Charm would be the first option, but Xerneas does not learn the move in the main series games.
In order for Xerneas to be viable, Niantic would likely need to give the Legendary its signature move. Geomancy, which is the signature, is a unique Fairy-type move that could add Xerneas to the PVP tier list.