Dear Divya, The tense in both terms brings out the difference.
"Passed out year"
This term is applicable to students who have already completed their course. They might have finished the course in a preceding year.
"Passing out year"
Refers to students who are currently pursuing their education and are expected to complete the course in the next 1-2 academic years.
Note: If the academic course you are pursuing will be completed in 2023, then the passing out year will be 2023.
Thanks,
Dinesh Divekar