I have been farming the Keywardens since the release of Diablo 3 1.05. I have tried many ways to find the fastest possible way to farm each one. This guide shows my method of farming the act 1 Keywarden the fastest way I have found.
Start any quest where you have the ability to portal to both Fields of Misery and Highland Crossings. Pick a Monster Power level suitable for your gear level. I am able to farm on MP4 efficiently. Start by portaling to the Highland Crossings. The place we are looking for is the Cave of the Moon Clan. This cave will spawn enough elites and a resplendent chest to get you 5 stack of valor. Cross the bridge and head North for about 10 second. There is only 2 major places this cave spawns so you should be able to find it quickly. If you do not find it by heading North, you will need to turn and head Southwest. After about 10 seconds you will turn to head straight West. You will find the cave along this path most of the time. Once you near the West wall you will turn and head Northeast. It should take you no more than about 45 seconds to find the cave unless you find an elite pack on the way. Reference my screenshot for the correct path. Follow the orange path. The red X's indicate where the cave spawns. Kill any elite pack you find along the way and then enter the cave. After clearing the first and second level you will be 5 stacked. You should kill 1-3 elites on the first level. You will kill another 2 elites on the second floor and a resplendent chest which will yield 5 stacks of valor.
Next you will portal back to town. The act 1 Keywarden is located in the Field of Misery. Portal to the Field of Misery and begin your search. There doesn't appear to be any rhyme or reason where the Keywarden spawns. I have found him virtually everywhere on the map. I typically do this run in about 6-8 minutes. Ill kill 5 to 8 elites and usually a goblin or two.