누구세요 is compose of
누구 + 이(다) + 시 + 어 + 요
누구 means "who"
이다 means "to be"
시 is honorific infix
어 is end ending suffix
요 is honorific suffix
누구예요 is compose of
누구 + 이(다) + 어 + 요
누구 means "who"
이다 means "to be"
어 is end ending suffix
요 is honorific suffix
The difference between "누구세요" and "누구예요" is the honorific infix 시.
It expresses the social superiority of the subject of the sentence to the speaker. As applied to adjectives, it expresses the relationship between the modified noun and the speaker.
Thus, 누구세요 is more honorific expression of 누구예요.
누구십니까?(most formal) -> To much older person than you.
누구세요?(more formal) -> To a little older person, To same aged person than you
누구예요?(a little formal) -> To same aged person, To younger person than you
누구야?(informal) -> To much younger person than you.