barm vs leaven

barm and leaven both are nouns.

Word	Noun	Adjective	Verb	Adverb
barm	Yes	No	No	No
leaven	Yes	No	Yes	No

As nouns, leaven is a hypernym of barm; that is, leaven is a word with a broader meaning than barm:

barm: a commercial leavening agent containing yeast cells; used to raise the dough in making bread and for fermenting beer or whiskey
leaven: a substance used to produce fermentation in dough or a liquid

Other hypernyms of barm include leavening.

barm (noun)	leaven (noun)
a commercial leavening agent containing yeast cells; used to raise the dough in making bread and for fermenting beer or whiskey	an influence that works subtly to lighten or modify something
	a substance used to produce fermentation in dough or a liquid

barm (verb)	leaven (verb)
	cause to puff up with a leaven