XenServer is based on the open-source Xen hypervisor. XenServer 7.5 runs on the Xen 4.7 hypervisor and adds support for "live-patching" of the Xen hypervisor, allowing issues to be patched without requiring a host reboot. Xen 4.7 also includes various performance improvements, and updates to the virtual machine introspection code (surfaced in XenServer as Direct Inspect).
XenServer uses paravirtualization and hardware-assisted virtualization, requiring either a modified guest OSs or hardware assisted CPUs (more commonly seen as its less restrictive and hardware assisted CPUs like Intel VT/AMD-V have become standard). The device drivers are provided through a 64-bit Linux-based guest (CentOS) running in a control virtual machine (Dom0).