Most boards put out 3.5Vp-p video level (some boards as low as 2.5V and [very rarely] 4V+) and 5V sync. Professional video equipment is designed to work with 0.7Vp-p and maybe up to 1V max video level, and sync level is fine at 5V. PVMs probably don't have much of a problem with hot video, but it'll play much nicer if you give it what it expects.
The contrast knob on your arcade monitor is just a video input level control, but the one on the PVM isn't made for this sort of range.