Do we need any visa for flight from Shanghai to Prague (MU 785) with 1 stop in Xi An?

Dear madam / sir,
We are travelling from Prague to Auckland NZ on 22nd December via Shanghai (flights MU 708 and MU 779). For 1 day we plan to visit Shanghai. We want to ask for free transit 24 hours directly at Shanghai airport after arrival, I hope this will not be a problem.
My question is about our flight back, on 10th january we fly back from Auckland to Shanghai (MU 780) and then from Shanghai to Prague (MU 785).
On this flight MU 785 to Prague we have 1 stop, I think only technical one in Xi An, please should we have a touristic l for this flight (MU 785)? We will not leave airports neither in Shanghai nor in Xi An during this flight. Or transit ones? Or we do not need entry permit for this back flight at all?
If you need, I can send you our flight ticket details.
Thank you very much in advance for your help :)
We need to be sure we will not have problems with this 😊 on our way back form holiday.
Have a nice day :)
Renata Bodiova
Slovakia