Understanding the Relieving Process
You say you have served your notice period. If you have completed all the relieving formalities properly, there is no doubt your HR will give you your relieving letter, keeping it in flowers. If they are not providing it, talk to them. Ask what other formalities need to be completed. Maybe your handover is not completed, or there may be other pending issues. Talk to them and reach an understanding. Complete everything accordingly.
Please remember, HR is a mediator and can process your relieving only after receiving clearances from all departments and your HOD, especially. HR is always a service support team to support employees and employers, not to put anybody into trouble. It is a party that considers all parties, excluding its own.
Hope Pandit Satish has also got his answer.
Regards,
Bharghavi