"Underpinning the class system is XP distribution. XP is distributed with an eye on how effective a player you are being for your team, not how awesome you are at headshots (which, incidentally, will only ever result in a one shot kill if you manage to land one on the very agile and fast moving ‘light’ body type)."
source - http://www.vg247.com/2011/02/28/stern-on-the-brink-in-an-fps-effectively-the-gun-is-you/
also stated in the same preview, Ed Stern touches on xp related to kills.
"We look at how to incentivise players. We give you much more XP for buffing your teammates than for kills, and we smother you in XP for actually doing the objective. In addition, the XP that we do award for kills is proportional to damage, so if you cause 90 percent of the damage to an enemy and someone else swoops in and delivers the final shot, you will get 90 percent of the kill XP.”