is there any direct train or bus from xian to tibet? or other ?

There are no direct express trains from Xian to Tibet. As I know, there are four trains from Beijing, Shanghai, Chongqing North and Guangzhou to Lhasa. The trains from Shanghai, Chongqing North and Guangzhou pass by Xian. So you can choose to take these trains to Tibet. And I didn't find any bus from Xian to Lhasa.