அவர் மாணவர் அல்லவா? -is the equivalent. Means, He is a student, is not he? or He is a student, right? அவர் மாணவர் இல்லையா? - Isn't he a student? Considering above as an example, Questioning with அல்லவா? Means, you know already he is a student and asking like 'he also a student, right? Questioning with இல்லையா? Means, Either you don't know or so far you were been thinking that he is a student and asking in a situation when you are coming to know he is not a student. And also questioning with இல்லையா?, you can use either in both situation above. Was that clear?