To sit an exam and to take an exam both mean the act of answering the questions. To pass an exam is to get good marks; the opposite is to fail an exam. Then you would have to take the exam again.

To take an exam means to participate in an exam or test, answering (or trying to answer) the questions. To pass an exam means to take the exam and receive a passing grade. You can take an exam and not pass it, if your score is too low. "Sit exam" has no meaning. To "sit in on an exam" would mean to be present during the exam while others take it, but not take it yourself. This is very seldom the case, but i have seen it.