Multiple transits in China

Jan, you did not mention where will you fly after Xi'an. This is quite an important detail in order to determine your eligibility. Never mind because I will assume that you will fly, after Xi'an, to a country different than Japan and mainland China.

If my assumption is correct and your entire transit time is shorter than 24 hours counting from your scheduled landing time in Beijing until your scheduled departure time from Xi'an, you are definitely eligible for 24 hour visa free transit and you do not need any kind of visa.

You have to be aware that you will perform all your immigration formalities in Beijing. China has very similar way on doing the immigration as it done by Schengen area or to be precise, all the formalities are performed at the first landing/airport in your case in Beijing. Immediately upon your landing in Beijing, the first thing that you will do is to approach 24/144 hour visa free transit counter where you will apply for a temporary entry permit to be allowed to cross the immigration, This application is free of any charge and here you will present your flight tickets. After a short check, your permit will be granted and you will proceed to the immigration and then to the gate for your domestic flight to Xi'an.

After you land in Xi'an, you will just proceed through security and immigration toward the gate for your flight to some third country. Basically, this is it in short and I wish you a pleasant journey.