Hardware: This includes the physical server components. Theya re memory, CPU, disk drives among other physical components.

Hypervisor: The Xen hypervisor is a thin layer of software (less than 50,000 lines of code that provides the core abstraction layer between hardware and operating systems.

Domain 0: Domain 0 is the Linux VM with higher priority to hardware than the guest OS's. Domain 0 is responsible for managing the network and storage I/O of all the guest VM's. Because of the use of Linux drivers in the Domain 0 hardware access is much easier since linux drivers for new hardware are generally easier to obtain over drivers for proprietary operating systems.

The Linux virtual machines use paravirtualized kernels and drivers, while the network and storage are thill accessed through the Domain 0, the vm still maintains fast access directly to the CPU and memory through Xen.

The Windows VM's also use paravirutalized drivers or are "enlightened" to use the AMD-V and Intel-VT virtualization capabilities. Like the Linux vm's (not in Domain 0) the Windows VM's use the Domain 0 to access network and Disks.