I think this is up to house-ruling... If you take the rulebook literally there is no experience awarded for vilains as you get the experience that is indicated on the villain (monster) card and no experience points are indicated on them.
If you play the missions like they are in the adventure book of Ravenloft, allmost all missions end after defeating the villain, this can only be an issue in the mission where you need to defeat strahd and his bodyguard and if you should kill klak before destroying his artefact.
I would say that the game becomes too easy if you play the regular game and award experience points, because it gives you the right to discard an encounter card..
I myself play keeping in mind that you are on the villains home teritory, so if there are questions about the rulebook, it is always in favour of the villain/monster..
If you should write missions yourself, you also have the right to write specific mission rules, so in that case it is up to you.
So basically this is all up to houseruling and finding the right balanse for your game.
I hope this is clear.
(sorry for the bad english)