This talk with discuss the design and implementation of a new type of hypervisor derived from the Xen code base. µ-Xen has been built and optimized for modern CPUs and chipsets, and thus assumes the presence of CPU and IO MMUs that are virtualization capable. µ-Xen borrows extensively from the production-proven and tuned Xen code base, but removal of support for older hardware and PV-MMU guests has enabled significant simplification of the code. µ-Xen supports optimizations in support of running large numbers of very similar virtual machines, through the support of a native 'vmfork' optimization and efficient re-merging of shareable pages. The primary goal of µ-Xen has been to run as a late-load hypervisor on an existing OS. It has a narrow and well-defined interface to the services it expects from the underlying OS, which makes it easy to port to other OSes, or to enable it to run on bare metal. During initialisation, µ-Xen can de-privilege the running host OS into a VM container, enabling it to establish itself as the most privileged software component in the system. Thus, µ-Xen enforces the privacy and integrity of itself and VMs that it is running, against a faulty or malicious host OS, while co-operating with the host OS on the actual allocation of physical resources.