Traveling in Tibet, you need not only China Visa but also Tibet permit.
There are 3 kinds of permits, Tibet Entry Permit (TTB permit), Tibet
Travel Permit(PSB permit) and Military Permit.
TTB Permit - Tibet Entry Permit
When you fly into Lhasa you will need to show this permit when checking in at the airport. Also, in order to buy a flight to Lhasa in the first place you will need to show this permit. TTB permits are also needed by groups traveling by Land Cruiser, but this will be arranged by the travel agency organizing the trip.
PSB Permit - Alien Travel Permit
If you want to "restricted" areas in Tibet, you will need an Alien travel permit issued by the PSB (Public Security Bureau) of Tibet. The Alien travel permit is also sometimes referred to as the PSB permit.
How to Apply for Tibet Entry Permit?
All the holders of ordinary passports are required to apply for a Tibet Entry Permit if they plan to travel in Tibet. As a Chinese international travel agency, we can easily obtain travel permits on your behalf. Simply send us the following information:
1) Your full name;
2) Your gender;
3) Your date of birth;
4) Your passport number;
5) Your nationality;
6) The first page of a valid passport;
7) A copy of their Chinese visa by fax or by email;
8) The occupation (Foreign journalists and diplomats are not allowed to go to Tibet as a tourist)
* Foreign tourist must buy tickets and travel in Tibet in a designated tour group.