When you arrive in Beijing from HK, you have to pass all of the courses for entering an international city, so at least one hour is needed. Then, for check-in flight at T2 to Xi'an, 30 minutes is enough. If you use T3, you should finish checking in 45 minutes before the plane takes off.

As for terminal transfer, the flights from HK to Beijing will land at T2 or T3, both of which have the flights to Xi'an. So, you'd better check which airlines you will use.

For your reference, there is dirct flight from Hong Kong to Xian. If you do not need to go to Beijing, you can fly from Hong Kong to Xian directly.