Pokemon X&Y has the unique problem compared to previous generations, that it can't be data-mined yet. In previous generations it is possible to check the data itself and figure out the EV's.
In the case of X&Y, they have to test the EV's manually. Here's the best way I can think of, but by no means fast.
- Save the game.
- Take a Pokemon with 0 EV's and no items or Pokerus, to a newly "discovered" Pokemon. Battle and win.
- Use a Reset Bag in Core Training.
- Once the Reset Bag is beaten up, you will see exactly how many EV's the Pokemon has lost. This is the EV yield of the battle that was just fought.
- Soft reset to get the Reset Bag back.
From Terlor
This answer is only an addition to Malachite's answer.
There is data on Xerneas' and Yveltal's EV yield.
According to Serebii they both give 3HP points (and so does Zygarde).
Source and source.
it seems odd for all 3 to give out HP evs, so if this is wrong let me know in the comments and i'll convert it to a comment.
