The Xen virtual machine monitor can run in HVM (hardware virtual machine) mode, using Intel VT-x or AMD-V hardware x86 virtualization extensions. This means that instead of paravirtualized devices, a real set of virtual hardware is exposed to the domU to use real device drivers to talk to.
QEMU includes several components: CPU emulators, emulated devices, generic devices, machine descriptions, user interface, and a debugger. The emulated devices and generic devices in QEMU make up its device models for I/O virtualization.
Xen-HVM has device emulation based on the QEMU project to provide I/O virtualization to the VMs. Hardware is emulated via a patched QEMU "device model" (qemu-dm) daemon running as a backend in dom0. This means that the virtualized machines see as hardware: a PIIX3 IDE (with some rudimentary PIIX4 capabilities), Cirrus Logic or plain VGA emulated video, RTL8139 or NE2000 network emulation, PAE, and somewhat limited ACPI and APIC support and no SCSI emulation.