The simplest and least expensive backflow preventer suitable for most residential sprinkler systems.  It is self draining and has very low maintenance requirements, though it is not suitable for backflow prevention in many circumstances, including scenarios of continuous water pressure.  This backflow preventer does not need to be tested annually.  

The PVB is an evolution of the AVB, intended to allow for scenarios of continuous water pressure.  Requires proper conditions to work, including the ability to be placed several inches higher than all subsequent system components, as well as there being no further down-line valves, nor any back pressure, as they are designed only to protect against backsiphonage, but not backpressure.  PVBs must be tested annually by a certified backflow preventer tester.  

the PRZ represents the best protection, protecting against both backpressure as well as backsiphonage.  This type of backflow preventer is required when there’s a risk of cross contamination with the city water supply, and in continuous pressure applications.  RPZ assemblies are frequently used in commercial systems.   RPZ assemblies must be tested annually by a certified backflow preventer tester.

The D backflow preventer is no longer allowed to be installed under current code, but can be maintained if it was installed under the previous code.  It was used to protect from backflow situations where there is a low hazard of cross contamination with drinking water or continuous pressure applications.  It must be tested annually by a certified tester.