Nested virtualization (i.e. running hypervisors on top a VM in Ravello) is currently officially supported and tested using RHEV/CentOS6.5, Ubuntu 14.04 and ESXi.
Starting September 2015 enabling nested virtualization can be done by simply turning on the 'Allow nested virtualization' flag on the virtual machine.
Via the UI: select the VM (in the library or in an application) -> Select the System tab -> check the 'Allow nested virtualization' flag. Do not forget to Update changes if you had done it on a VM in a published application.
Via API: set the allowNested flag to true in the VM design section. For more information, see our API documentation here: http://www.ravellosystems.com/developers/rest-api/images
Note that the VM library in Ravello includes three preconfigured, nested virtualization enabled machines - the "Ubuntu Server 14.04.1 with qemu-kvm", the "Xubuntu Desktop 14.04.1 with qemu-kvm" and the 'Empty ESX' VM.