An easy and inexpensive way to watch Pokémon is by downloading the Pokémon app on iOS and it's completely free. The app provides a rotating list of shows taken from each period of the series, and there is always one available as well. It's a great starting point to enjoy watching Pokemon XY.
If you're willing to spend more, iTunes has the largest collection of Pokémon content on this list. It is the best way to enjoy all your pokemon series on a go with complete access to background interviews, behind the scenes, and many more. I personally recommend iTunes, if you want to enjoy every bit of Pokemon XY series.