Which group do the Proximity sensors belong to?
You've got things upside-down:

Proximity detectors aren't a subgroup of PIR sensors. But it's possible that there are PIR sensors which are used as proximity detectors.

Point being, multiple sensor technologies can be used as proximity detectors. For instance:


IR photoreflector (IR LED and IR photodiode)
capacitive sensor
ultrasonic rangefinder
IR rangefinder (an IR LED and a linear IR CCD, see Sharp GP series stuff)
even a Hall effect sensor could be used as a proximity sensor if you're sensing the proximity of a particular magnetic target.


etc.