XenServer is based on the open-source Xen hypervisor (XenServer 7.0 now runs on the Xen 4.6 hypervisor, provides GPT support and a smaller, more scalable Dom0 based on Centos 7.2). XenServer automatically scales the amount of memory allocated to the Control Domain (Dom0) based on the physical memory available.
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).