hollo vs howl

hollo and howl both are nouns.

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

As verbs, howl is a hyponym of hollo; that is, howl is a word with a more specific, narrower meaning than hollo:

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

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