The Chinese government does not issue visa for individual traveler to visit Tibet. Therefore, what you have to understand is that the Chinese embassy will only issue visas to travelers on group tours. You require two documents, if you wish to travel in Tibet. One is the Chinese visa, which you can apply in Chinese embassy in Kathmandu or your country and another is the alien's travel permit that is issued by Tibet tourism bureau. Non-Chinese passport holders, including Taiwan, Hong Kong and Macau among others must obtain an alien's travel permit as well as a valid passport and visa. You can obtain visa from the Chinese consulate in your country. The Chinese government mainly issued group permits, which can be proceeded through Chinese tour operators.