What is the difference between PAM and PCM?
PAM encoding uses the physical amplitude of the sample as the final modulation (as seen on this page); i.e. it is an analogue modulation technique (the amplitude used for modulation is the actual sampled value, not the nearest approximation as is used in PCM, although it can be bounded).

Put another way, it is not (as part of the modulation) converted to a digital data word. 

This modulation method forms the basis of QAM

When used as an analogue method, there is no inherent quantisation noise.

PCM takes the same thing a step further by requiring a step size (which may not be linear across the range) and then encoding that as a digital value (which will have a certain number of bits per sample); this will always introduce quantisation noise. 

PCM data is widely used in the telephone system in most countries.