Hey There!
The main difference between these 3exams is the level of the questions that are asked.
The syllabus for all there exam is similar.
NEET and AIIMS are both based on 12th Class Syllabus.
In the board examination, you are supposed to write essay type answers to th questions. There are a few MCQs as well. There is no negative marking for wrong answers and you are supposed to write elaborated answers.
In NEET and AIIMS, the paper is fully MCQ(Multiple Choice Question) based. And there is negative marking as well.
Each wrong answer will cost 0.25marks, which means 4wrong answers deduct 1marks.
For NEET and AIIMS, you have to have clear concept and be through with all the topics, as you will have to chose the most correct answer from the given choices.
In Physics, you have to be quick in solving equations and finding out the right answer from the given options.