face veil vs niqab

face veil and niqab both are nouns.

Word	Noun	Adjective	Verb	Adverb
face veil	Yes	No	No	No
niqab	Yes	No	No	No

As nouns, niqab is a hyponym of face veil; that is, niqab is a word with a more specific, narrower meaning than face veil:

face veil: a piece of more-or-less transparent material that covers the face
niqab: a face veil covering the lower part of the face (up to the eyes) worn by observant Muslim women

face veil (noun)	niqab (noun)
a piece of more-or-less transparent material that covers the face	a face veil covering the lower part of the face (up to the eyes) worn by observant Muslim women