Nested virtualization provides an extra layer of virtualization to enhance security with fairly reasonable performance impact. Usercentric vision of cloud computing gives a high-level of control on the whole infrastructure, such as untrusted dom0. This paper introduces RetroVisor, a security architecture to seamlesslyrun a virtual machine (VM) on multiple hypervisors simultaneously. We argue that this approach delivers high-availability and provides strong guarantees on multi IaaS infrastructures. The user can perform detection and remediation against potential hypervisors weaknesses, unexpected behaviors and exploits.