unbeknown vs unbeknownst

unbeknown and unbeknownst both are adjectives.

Word	Noun	Adjective	Verb	Adverb
unbeknown	No	Yes	No	Yes
unbeknownst	No	Yes	No	Yes

As adjectives, unbeknown and unbeknownst are synonyms defined as:

unbeknown and unbeknownst: (usually used with `to') occurring or existing without the knowledge of

As adverbs, unbeknown and unbeknownst are synonyms defined as:

unbeknown (adjective)	unbeknownst (adjective)
(usually used with `to') occurring or existing without the knowledge of	(usually used with `to') occurring or existing without the knowledge of

unbeknown (adverb)	unbeknownst (adverb)
without someone's knowledge	without someone's knowledge