The official movie website has updated with information on the feature film's opening theme and the short's ending theme. Translations follow:

The new anime opening theme and ending theme that premiered in the anime's Thursday May 29th broadcast will be the opening theme to the feature film "Diancie and the Cocoon of Destruction" and the ending theme to the short "Pikachu, What Kind of Keys are These?"

The Cocoon of Destruction and Diancie Opening Theme - "Megavolt" (メガV)
Pokémon the Movie XY
"The Cocoon of Destruction and Diancie" Opening Theme
"Megavolt" (メガV)


Lyrics: Yusuke
Composition: Hirokazu Tanaka
Remix Arrangement: Mitsuki Shiokawa
Original Arrangement: CHOKKAKU
Vocals: Yusuke (Sony Music Records)


The popular opening theme "Volt" mega evolves as well! An even more powerful remix makes its debut!

"Pikachu, What Kind of Keys are These?" Ending Theme - "Peacsmile!" (ピースマイル!)
Pokémon the Movie XY
Short "Pikachu, What Kind of Keys are These?" Ending Theme
"Peacsmile!" (ピースマイル!)


Lyrics: PA-NON
Composition and arrangement: Hirokazu Tanaka
Vocals: J☆Dee’Z with Pikachu (Sony Music Records)


J☆Dee’Z and Pikachu sing an ending theme that will bring smiles to your faces! There's a cute and fun dance that goes with it too, so dance along!

Source: http://www.pokemon-movie.jp/news/?p=1251