The short form of 'who have'. You meet it mostly inside a longer sentence, joining two ideas about people: 'Students who've finished can go home.' It is much less common than 'who's', and it is really only used when 'have' is a helping verb before a past participle — 'who've seen', 'who've been', 'who've done'. When 'have' means to own or to possess, we do not squeeze it: we say 'people who have three children', not 'people who've three children'. It sounds normal in speech and in relaxed writing; in careful formal writing, write 'who have' in full. In fast speech it can shrink so much that it sounds almost like 'who' alone, which is why learners often miss it when listening.