Why not just make it harder for people to AFK in pvp as opposed to nerfing the xp?
1. Increase how often I can kick people in PVP instances to once every 20-30 minutes.
2. Auto-Kick players who are out of combat for more than 3-4 minutes.
3. Give an additional amount of xp to 1st place teams and a smaller amount of xp to 3rd place teams.
because if you don't do something to remove the afkers, people will still just afk pvp. The XP rate doesn't have to be amazing to have people afk-ing it. It just has to have people willing and able to AFK it.

Let me know your opinions guys.