analog PIR is typically a differential signal between two receivers. The receivers are placed behind a fresnel lens (where the lenses are designed to point into each of the sensors). So when an IR source (like a person) walks in front of the sensor it creates a large spike, because they are being compared against a low IR area.

There is a form of PIR sensing where it behaves like a thermal camera at a very low resolution (sometimes as low as 2x2 pixels)