The SPV system works the same way as a monotube rear shock, they need positive pressure in the damper to prevent them pulling a vacuum on the seal-head side of the piston.
Other damper designs use a set of compression pistons/shims to create the positive pressure under compression and negate the need for extra air pressure. On some designs (like manitou TPC) virtually all the compression damping is done by the second piston (or pair of pistons), which means the moving piston cannot pull a big enough pressure difference to cavitate.
Cavitation can be hard to pick when riding, but there are some circumstances where it's very obvious. One would be a friend who landed a jump and was catapalted straight over the bars and bounced off a tree.
His rear shock compressed faster than the valving would let the oil flow, so the oil stretches out to a gas bubble which then implodes. Because there is little oil to move through the piston and damp the motion you get a very fast compression, followed by a very fast rebound.