A guardian farm is a contraption, that automatically spawns and kills guardians, and puts the items into chests.
Guardian Spawning conditions[]
Guardians spawn inside the 58x58 area of the monument from Y level 39 to 61. They require a 1x2x1 (x,y,z) area to spawn. When there is direct sunlight access to the water source the chance of them spawning is 10 times lower. That's why using a roof drastically improves the rates. It's also really good to use a non-solid block (glass, glow stone, slabs, or red stone blocks) since the new mob spawning AI starts off at a transparent block so it raises the rates quite a bit from my testing.
Processing Guardians[]
Once the guardians spawn they need to be killed as quickly as possible so I recommend having blocks next to each water source in order to get a downwards going stream. They need to drop 35 blocks in order to be killed. (34 will kill most of them but not all of them). A good video that explains these things and more is this one: https://www.youtube.com/watch?v=s0FiZ51A6Qg
(The information from that video should be extracted to this page)
Current fastest farm designed:
☀https://www.youtube.com/watch?v=R2wflKaKV5I
Guardian XP Drops[]
Guardians are one of the best ways to get xp in the game. Guardians drop 2 xp orbs: a 7 orb and a 3 orb (10 overall), that makes them a great source of xp because they give a lot of xp but not a lot of orbs, that makes them great because that means they have a much higher max xp per second rate. Guardian Xp farms can be a big project but they are a very good source of xp mostly in more advanced worlds or servers.