PM vs Prime Minister

PM and Prime Minister both are nouns.

Word	Noun	Adjective	Verb	Adverb
PM	Yes	No	No	No
Prime Minister	Yes	No	No	No

As nouns, PM and Prime Minister are synonyms defined as:

PM and Prime Minister: the person who holds the position of head of the government in the United Kingdom

Other synonyms of PM include premier.

PM (noun)	Prime Minister (noun)
an examination and dissection of a dead body to determine cause of death or the changes produced by disease	the person who holds the position of head of the government in the United Kingdom
modulation of the phase of the carrier wave
the person who holds the position of head of the government in the United Kingdom