After a cloud computing decade, the user-centric, fully interoperable, multi-provider cloud remains a mirage. In currently deployed architectures, "horizontal" multi-cloud interoperability limitations come on top of "vertical" multi-layer security concerns. In this paper, we argue that an architecture with a hybrid design could be a viable solution. Indeed, we present a new virtualization architecture combining micro-hypervisor (MH), nested virtualization (NV)and component-based hypervisor (CBH) paradigms. Leveraging NV interoperability and legacy support, the architecture provides to users a transparent federation of multiple-provider resources. We also adopt a MH including CBH-like modules as NV lower-layer hypervisor to achieve both a minimal TCB and to enable users to directly control hypervisor components managing their resources.