Nobody is really sure at this point, but they're most likely Kalos' event legendaries. Game Freak will probably release them at some future date. If you hacked the game, you'd be able to get them, but as far as I know, hacking 3DS games is really hard.
Update (June 2014)
Those Pokemon are definitely real, they just cannot be obtained yet. Tests on the GTS confirm that the Pokemon do indeed exist within X and Y's code.
Diancie was confirmed by Nintendo a few months ago, along with a movie featuring it that will be released soon. The latest CoroCoro issue also revealed Mega Diancie, so we can expect Diancie to make an appearance of some sort in the upcoming games.
We have not heard anything from Nintendo or any other reliable source about Volcanion or Hoopa yet, however there is a good chance they will be confirmed soon.
