PM vs head of state

PM and head of state both are nouns.

Word	Noun	Adjective	Verb	Adverb
PM	Yes	No	No	No
head of state	Yes	No	No	No

As nouns, head of state is a hypernym of PM; that is, head of state is a word with a broader meaning than PM:

PM: the person who holds the position of head of the government in the United Kingdom
head of state: the chief public representative of a country who may also be the head of government

Other hypernyms of PM include chief of state.

PM (noun)	head of state (noun)
an examination and dissection of a dead body to determine cause of death or the changes produced by disease	the chief public representative of a country who may also be the head of government
modulation of the phase of the carrier wave
the person who holds the position of head of the government in the United Kingdom