PCM is a type of pulse modulation like PAM, PWM or PPM there is an important difference between them PAM, PWM or PPM are "analog" pulse modulation systems whereas PCM is a digital pulse modulation system.
That means the PCM output is in the coded digital form, it is in the form of digital pulses of constant amplitude, width and position.
The information is transmitted in the form of "code words". A PCM system consists of a PCM encoder (transmitter) and a PCM (decoder) (receiver)
The essential operations in the PCM transmitter are sampling, quantizing and encoding. All these operations are usually performed in the same circuit called as analog to digital converter.
The transmitter converts the message signal X (+) into a series of coded pulse and over communication channel.
Block diagram of PCM encoder/transmitter