Islamabad to Beijing
There is direct flight on every Thursday and Sunday. Otherwise, you need to transfer at Urumqi, Hong Kong, Dubai, Bangkok, or Karachi. The direct flight takes only 5 hours and 35 minutes.
It departs from Islamabad at 23:30 (Islamabad local time) and arrives in Beijing at 07:05 (Beijing time). Pakistan International airlines, PK852.

Beijing-Xian, more than 20 flight are provided everyday for your choice; the earliest one departs from Beijing at 07:30 and the last one at 22:05. Flight duration: 1h 50 minutes to 2h.