Pulse modulation:

The carrier is in the form of train of periodic rectangular pulses.

It can either analog or digital

In analog modulation width or position of rectangular carrier pulses is changed in accordance with modulating signal.

E.g. Pulses amplitude mod, (PAM), pulse width mod (PWM) and pulse position mod (PPM)

PAM:

Amplitude of a constant width, constant position rectangular carrier is varied in proportion with instaneous magnitude of modulating signal.

PWM:

The width of carrier pulses is made to vary in proportion with the instantaneous mag. Of modulating sig. Also known, pulse duration/length modulation, i.e., PDM or PLM

PPM: In PPM amp and width of the pulse is kept constant but position of each pulse is varied in accordance with amplitude of sampled values of modulating sig, the position of pulses is changes w.r.t. position of reference pulses.