bed linen vs bed sheet

bed linen and bed sheet both are nouns.

Word	Noun	Adjective	Verb	Adverb
bed linen	Yes	No	No	No
bed sheet	Yes	No	No	No

As nouns, bed sheet is a hyponym of bed linen; that is, bed sheet is a word with a more specific, narrower meaning than bed linen:

bed linen: linen or cotton articles for a bed (as sheets and pillowcases)
bed sheet: bed linen consisting of a large rectangular piece of cotton or linen cloth; used in pairs

Other hyponyms of bed linen include case, pillow slip, pillowcase, slip, pillow sham, sheet.

bed linen (noun)	bed sheet (noun)
linen or cotton articles for a bed (as sheets and pillowcases)	bed linen consisting of a large rectangular piece of cotton or linen cloth; used in pairs