H, you can take a train or flight. Train fare is about 210 yuan for hard sleeper and 320 yuan for soft sleeper.It will take about 17 hours to get to xian.
Actually i think air is more cost efficient. The discount air fare is about 500--600, if you have a pre-booking. The earlier, the more discount.
Check this page for the air fare:
https://www.travelchinaguide.com/china-flights/