The auto ignition point of cardboard where it will ignite and catch fire is around 450 F so you can place a Pizza box inside the oven to keep the Pizza warm however make sure there's no plastic in the box underneath the pizza etc to avoid it melting.
Preheat the oven to 450 F and then before putting the pizza and pizza box in the oven turn the oven down to 150 F which will keep the pizza warm.
You could also just take the pizza out of the box and wrap the pizza in Aluminum foil and place it on a pizza pan to keep the pizza warm and then put it back in the box when ready to serve or take somewhere with you.
The aluminum foil will help hold the heat inside the pizza keeping it warm.