A little confused…… but I find on the internet that both direct flights and conjoint flights are available from Islamabad to Beijing. And direct flights are available only on Thursdays and Sundays. Islamabad – Beijing PK852 2330/0705

From Beijing to Xian, there are many flights available daily. Anyway, I suggest you contact some booking agency directly and generally they can offer you more details including the airfare details provided you have the exact travel date.