do i need a visa?

Yes, you will. It is important to know that your flights from Seoul to Xi'an and from Xi'an to Hong Kong have to be literally nonstop flights because to be eligible for 144 hour visa free transit in Xi'an area, you are not allowed to have additional landing of any kind within mainland China. If you have flights of this type, you will be eligible to enjoy up to 6 days in Xi'an area absolutely visa free. Please, just have in mind that this is a 144 and not 72 hour visa free transit. 72 hour visa free transit has been replaced in Xi'an by more generous 144 hour visa free transit.