In my opinion nonviolent-solution XP should be equal or superior to the max gain for simply killing everything, and should disable XP gain should you choose to kill anyway.
Given that you will need to engage in combat anyway, and probably in more challenging situations than the ones that you can avoid through dialog, being able to complete nonviolent solutions in addition to violent ones requires a stronger character and/or player than one that can only complete violent solutions, which is why it deserves at-least-equivalent reward. Attempting to do both for the same encounter however amounts to trying to have your cake and eat it too, which should be avoided in design.
