Like the previous generation, Pokémon X & Y bring a few changes to how Experience Points are distributed

EXP. Share

The most notable change is with the item EXP Share and how the experience is distributed. When you get EXP. Share, you learn that it's now a Key Item and, when turned on, it will give every unfainted Pokémon experience. However, unlike past generations, it's no longer cut by the amount of Pokémon that were in battle. Instead, all Pokémon who were in the battle will automatically get 100% of the experience, but those which weren't in battle will receive 50% of the experience

Boosts

In addition to the previously standard boosts for traded Pokémon and foreign Pokémon,, as well as special powers such as O-Powers, a couple of new boosts have been included.

First, there is a boost if your Pokémon has a high affection in Pokémon Amie. If it has at least three hearts of affection, the experience it gains from each battle will be slightly increased.
The second boost is a rather unique one. If your Pokémon evolves by level and you've reached that particular level, it will start to gain boosted experience points from the battle. This will last until you evolve it.

Capture Experience

Within the Pokémon games, typically you have gained experience from defeating Pokémon, but never from capturing them. From Pokémon X & Y, however, this changes. In battle, should you capture one of the opposing Pokémon, you will gain Experience Points as if you had defeated it.

 
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