The difference is in the expectation. If you ask "don't you know it?", you expect the other person to know it already and are perhaps surprised that they may not. Example: if someone asked you, what is 2 + 2? You might say "Don't you know it (the answer)?!" The second phrasing, "Do you know it?" is a much more general enquiry and the person asking is genuinely curious if the other person knows something or not. Example: I can't remember in which year John Lennon died...do you know it?