Using KVM to run Xen guests without Xen
The inclusion of the Kernel Virtual Machine (KVM) driver in Linux 2.6.20 has dramatically improved Linux's ability to act as a hypervisor. Previously, Linux was only capable of running UML guests and containers. KVM adds support for running unmodified x86 operating systems using hardware-based virtualization. The current KVM user community is limited by the availability of said hardware.
The Xen hypervisor, which can also run unmodified operating systems with hardware virtualization, introduced a paravirtual ABI for running modified operating systems such as Linux, Netware, FreeBSD, and OpenSolaris. This ABI not only provides a performance advantage over running unmodified guests, it does not require any hardware support, increasing the number of users that can utilize the technology. The modifications to the Linux kernel to support the Xen paravirtual ABI are included in some Linux distributions, but have not been merged into mainline Linux kernels.
This paper will describe the modifications to KVM required to support the Xen paravirtual ABI and a new module, kvm-xen, implementing the requisite hypervisor-half of the ABI. Through these modifications, Linux can add to its list of supported guests all of the paravirtual operating systems currently supported by Xen. We will also evaluate the performance of a XenoLinux guest running under the Linux hypervisor to compare Linux's ability to act as a hypervisor versus a more traditional hypervisor such as Xen.