Excerpt VirtualBox "Hardware vs. software virtualization",
"VirtualBox's 64-bit guest support (added with version 2.0) and multiprocessing (SMP, added with version 3.0) both require hardware virtualization to be enabled. (This is not much of a limitation since the vast majority of today's 64-bit and multicore CPUs ship with hardware virtualization anyway; the exceptions to this rule are e.g. older Intel Celeron and AMD Opteron CPUs.)
You cannot nest HV on HV.
You can nest PV on PV,
"Oracle VM VirtualBox virtual appliance images for Oracle VM 3.1.1 server and Manager",