Hii. Himanshu...

Improvement exam and compartment exam are two different exams in CBSE .....

 Compartment exam are the exam . which given by  student who fail in one or more.....

One the other hand improvement exam..given by students who get the less percentile in their exam or they are not satisfied  with their results..as may the not able to get admission in their dream colleges...etc..

Compartment exam is a compulsory exam for the student who are fail in their exams to get marmsheet ...compartment exam held after few days of results declare ....

where as improvement exam is upto  candidate.... if they are not satisfy he/she may be or may not be apply for improvement exam and for improvement exam candidates have to gap for 1 year and give exam in next year..

Hope it will help!!