From Beijing to Xian, both by air and by train are convenient.
For the above attractions you mentioned, Terra Cotta Warriors would take you more than half a day, you can also go to Huaqing Hot Spring or Mt. Lishan, this would take you one day, and you can put Xian City Wall, Bell&Drum Tower, Big Wild Goose Pagoda, Muslim Street and Shaanxi History Museum all in one day. For Mt. huashan, you need a whole day since it is a little far from downtown Xian.

Enjoy your time!

Summerling. Thanks for your reply.
As I will be arriving Beijing in the late afternoon. Can I get the night train tickets to Xian when I arrive Beijing in the same day? What're the train timings and which Beijing train stations go to Xian? How much the train tickets? Or are there evening flights to Xian? Thank you.

After arrive at Beijing Airport, you could take the shuttle bus to Beijing West Railway Station directly, where there are night trains departing to Xian. However, I don't think you could purchase a train ticket on the spot. So, please book a ticket via some agent. Seems this website does have this service. The fare is about CNY400/soft-sleeper. Please use this website's trains search tool to get the schedule and rate in detail.

Alternatively, there are frequent flights from Beijing to Xian until to 21:00 at night. Of course, the fare is more expensive than taking a train. It will be about CNY800~CNY1200.

Mr.Jamie. Thanks for your reply.
How long is the shuttle bus ride to Beijing West Railway Station from Beijing Airport?

Assuming if I were to the flight to Xian from Beijing, how far is the distance from Terminal 3 to Terminal 2 as this would be a domestic flight? Thank you.