I have been down there twice for X-mas and once for NYE. The first time I made reservations in September and got reservations for Xmas pretty easily but the dates closer to NYE were sold out and the places I wanted to stay at. I ended up staying at the Abbey for one night.
The second time I made my reservations about 6 months in advance and had no problems for getting a room. The Blue Chairs and Casa Cupula tend to sell out early, if you're looking at the gay hotels. I ended up at The Mercurio both times.
PV is an incredible place to spend Xmas and NYE. Great town, really good local activities and an incredible fireworks display on NYE that goes on forever all over town.
If you are used to being here in the off season when it's easy to find a place to eat etc, it gets really crowded during this time for better or for worse. Restaurants, bars, beach all packed all the time.