Prime Minister vs PM

Prime Minister and PM both are nouns.

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

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

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

Other synonyms of Prime Minister include premier.

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