A blow off valve(BOV) releases the pressure of a forced induction system via actuation of your throttle. It slams shut when you go to wide open throttle(WOT) so you can build boost and opens when you release the accelerator or go to a vacuum condition.

A pop-off valve release at a predetermined level to help control surge and excessive boost.

A diverter valve is a BOV that is diverted back stream or simply routes the incoming air past the supercharger so you will not see boost when not needed under vacuum conditions and closes when you go to WOT.