If I use cpf to pay the full amount of HDB loan, what will happen when I reach 65 years old? Do I need to payback to cpf before I can start my cpf withdrawal?

You don't need to refund if you are not selling your HDB, regardless what age. Once you have FRS in your RA at 55, you can start your CPF LIFE monthly payouts at 65 (or postpone up to 70), and you can freely withdraw any amount in your SA and OA after 55.