good backflow questions, you probably know what I will say.
The PVB can be used in high hazard applications it protects against back siphonage, but not back pressure. The elevation in the sprinkler system can cause back pressure. That is why the PVB must be 12" above the tallest head and all piping.
The Double Check is ok for non Health hazard application or low hazard applications. It can handle back siphonage and back pressure.
Again it is low hazard. Check with local codes.
The RP is a great device that can handle high hazard, Back siphonage and back pressure. The RP Must be installed 12" above Grade. It absolutely can not be below grade. If water pressure is an issue, the RP can kill 10 to 12 LBS of your working pressure.
For more information on backflow issues check out
BackflowInformation.com