I'll assume you mean
帘 as in the simplified Chinese for
簾. In which case, strictly speaking:
帘 / 簾 is a cover for windows woven from bamboo slips (hence the traditional top radical) or fabric
帷 is curtains, in the sense of sides a tent woven from fabrics.
In practice, they both mean some sort of fabric material that obscures sight, and therefore usages have become mixed up. In this case, the
窗 in the word
窗帷 specifies its a window curtain, making it the same as
帘 / 簾.
帘 is actually a different character from
簾. It meant "a banner (of an inn or tavern)", and was flown outside such establishments.
帘 also happens to be the simplified Chinese character for
簾, this difference is not noticeable in a majority of online resources anymore.
Historically, 簾 is what court officials use while 帷 is what common folks use. This is Ancient China historical.