custody vs guardianship

custody and guardianship both are nouns.

Word	Noun	Adjective	Verb	Adverb
custody	Yes	No	No	No
guardianship	Yes	No	No	No

As nouns, guardianship is a hypernym of custody; that is, guardianship is a word with a broader meaning than custody:

custody: (with `in') guardianship over; in divorce cases it is the right to house and care for and discipline a child
guardianship: the responsibility of a guardian or keeper

Other hypernyms of custody include keeping, safekeeping.

custody (noun)	guardianship (noun)
holding by the police	the responsibility of a guardian or keeper
(with `in') guardianship over; in divorce cases it is the right to house and care for and discipline a child	attention and management implying responsibility for safety
a state of being confined (usually for a short time)