Tokyo - Xianyang - Shenzhen - Auckland

Hello!
I'm Spanish, flying from Tokyo to Xianyang with a 13 hour overnight stopover, then Xianyang to Shenzhen with a 3 hour stopover, and then Shenzhen to Auckland.
Since I have no time for any tourism, I thought I could just stay at the airport overnight in Xianyang. Of course, I won't be leaving the airport in Shenzhen at all, since I'm flying with the same airline all along (Hainan Airlines).

Do I need a Visa for my second stopover? Either way, which type am I most eligible for?

Thank you in advance!