Creds to Plexa from Teamliquid who had no problems with me reposting this.
From a Protoss perspective, PvZ was drasitcally altered by patch 1.1.2. Many players found themselves struggling with this matchup, despite being fine pre-patch. What caused this change isn't entirely clear - did Zerg's learn how to play properly? Or was the +1 Roach range really that significant? Arguably, it was a mix of both, but that isn't important. What is important is that now many Protoss players do not understand this matchup and, without doubt, Protoss do not understand the lategame of this matchup in the least.
This thread hopefully will change that. I would hope that any high level players would read this and agree with what is written here while lesser players might learn a thing or two from it. At the very least it can be cut and converted into something useful for liquipedia. The purpose of this thread is to educate, not cause a balance riot, nor to give the latest build orders for the matchup. Hopefully it will explain the concepts and themes running through the early, mid and late game of this matchup. It's unlikely that if you are 2k+ Diamond that you will gain anything from this thread, except maybe the comments about upgrades and the late game.
Maybe I'll add some pictures to this tomorrow, this turned into quite a wall of text!
For the purposes of this thread, early game refers to the period of the game where Zerg is trying to establish his economy while the Protoss is trying to respond to that. This is characterised by the Zerg taking his natural expansion, beginning his tech, and building up a large number of drones (or trying to). Once this foundation is established, the Zerg player is able to begin army production. While Zergs can 7RR, 5RR, baneling all-in, 6pool etc. these builds are not being considered here. On the whole these builds are shut down fairly easily by Sentries and appropriate cannon use. Further, macro games are where the strengths of Zerg shine so many Zergs are eager to play the economy game.
The Protoss early game is primarily concerned with not falling behind. Just leaving the Zerg to his own devices while taking a slow expansion off of 3 gateways is a sure fire way to lose to any competent Zerg (your economy will be too weak to compete, and he will eventually overpower you). Before the game begins you should have decided how you want to respond to the inevitable Zerg expansion - either 15 Nexus or Pressure.
The 15 Nexus option pretty much eliminates the early game phase of the game. Both you and the Zerg will be focusing on economy and neither player should be able to break the other (assuming you're responding correctly to any all ins). The only important thing to remember about this build is that scouting is essential - either via 2 Stargate Phoenix or Hallucination. Hallucination favours a ground based army follow up while Phoenix gives you the option of harass. The information you want to get from scouting is whether or not he has taken a really fast third and what his lair tech choice is (spire vs hydra den). This will be dealt with in the mid game section.
If you do not 15 Nex, then you must pressure the Zerg. The reason is simple, 15 Nex allows you to keep your probe production matched with his drone production. If you don't expand at 15, then he will have more drones since he will be powering all those larvae into drones. Pressuring the Zerg prevents that - he has to use those extra larvae on units, and not drones, which keeps his drone production in check and means you aren't falling behind. Of course, you are expanding behind your pressure (generally) and this allows you to transition into the midgame on even footing.
There are two key units in the Protoss early game - the Stalker and the Sentry. While these units are the most important, Zealots also have their place. I will devote the next few paragraphs to them and their role in this phase of the game before expanding on the types of pressure builds you can implement.
The Stalker is the strongest early game unit in this matchup - on either side. It's speed is unparalleled, until lings get speed. As such Stalkers are able to kite Zerg units very effectively and thus will be the core of any pressure attempts. (Zealot pressure is obsolete now due to Roaches). Their effectiveness peaks when you are able to get blink and pressure the Zerg with 3-4 gateways - ala Liquid`Nazgul. If you don't get Blink, then simply using hit and run micro (like Dragoons from Brood War) will generally be very effective. The most important thing to remember when pressuring with Stalkers is that you CANNOT lose them.