As Wazig noted, XP is granted when you cap. You may even be granted xp on HKs, although it is a small amount. This is why it is not wise at lower levels to BG to level if your intention is to level quickly. It is also the reason ( low xp gains) why many people have started making toons and quickly leveling them to level 10 getting into BGs with fully enchanted and full heirlooms and farming levelers. And as soon as they get to 19 or 20 just start all over again. These people consider themselves twinks because they have enchanted gear. Unfortunately, this lengthens our queue times as it takes them out of our pool.
/cheers