When the third part of the quest is being completed, you can give 10% of your xp, a level-based amount of gold or "supposedly" socket an epic gem in a statue that the quest giver gives you. When socketing the epic gem, no option to give the statue pops up, only experience or gold.
Even though the text says you will be giving up 10% of your experience(not total but of the current level) it in fact takes 90% of current level progression experience and leaves you with 10%.
If you want to keep most of your XP wait to turn this in until immediately after you level, the actual points lost are very minimal. It is even possible to earn some experience for completing the quest by doing so.