All overseas participants, except Chinese citizens, need a Chinese Visa to travel to China. You may find details about obtaining a Chinese Visa on the following website http://english.gov.cn/services/visitchina . Any questions regarding visa application can be directed to the Chinese Visa Application Service Center on the website http://www.visaforchina.org .
Depending on your country of origin, you may also need a Visa for the Special Administrative Region of Hong Kong, if you travel to Xi'an via Hong Kong. You may find visa information on the website http://www.immd.gov.hk/eng/services/index.html .