howl vs hollo

howl and hollo both are nouns.

Word	Noun	Adjective	Verb	Adverb
howl	Yes	No	Yes	No
hollo	Yes	No	Yes	No

As verbs, hollo is a hypernym of howl; that is, hollo is a word with a broader meaning than howl:

Other hypernyms of howl include call, cry, holler, scream, shout, shout out, squall, yell.

howl (noun)	hollo (noun)
a long loud emotional utterance	a very loud utterance (like the sound of an animal)
a loud sustained noise resembling the cry of a hound
the long plaintive cry of a hound or a wolf

howl (verb)	hollo (verb)
laugh unrestrainedly and heartily	utter a sudden loud cry
make a loud noise, as of wind, water, or vehicles	cry hollo
emit long loud cries	encourage somebody by crying hollo
cry loudly, as of animals