It is possible to run virtual machines (VMs) inside other VMs. This configuration is known as nested virtualization: 
Nested virtualization refers to virtualization that runs inside an already virtualized environment. In other words, it's the ability to run a hypervisor inside of a virtual machine (VM), which itself runs on a hypervisor.
With nested virtualization, you're effectively nesting a hypervisor within a hypervisor. The hypervisor running the main virtual machine is considered a level 0, or LO hypervisor, and the initial hypervisor running inside the virtual machine is referred to as a level 1 or L1 hypervisor. Further nested virtualization would result in a level 2 (L2) hypervisor inside the nested VM, then a level 3 (L3) hypervisor within that nested VM, and so forth.
Not all hypervisors and operating systems support nested virtualization.
Free Support Principle applies.
Nested virtualization is not a simple by-product of developing a virtualizer. Nested virtualization is not automatically offered as a feature and this is also true for various third party virtualizers. For example while the VirtualBox virtualizer has existed for years, the ability to run VirtualBox inside VirtualBox using Intel CPUs was only offered as a feature in v6.1 released in 2020.  This demonstrates that extra code is required for this functionality and that also implies a greater attack surface.
By mixing virtualizers -- for example by running VirtualBox inside the VMware virtualizer -- the attack surface is increased because the virtualizer code of both products is involved.
Running VirtualBox, KVM or Qubes inside Qubes is difficult and is not offically supported by the Qubes developers; this is unrelated to Whonix ™. To learn more about the current state of support, search the qubes-devel [archive] and qubes-users [archive] mailing lists for terms such as VirtualBox, KVM and/or nested virtualization.
VirtualBox inside VirtualBox
Perform these steps on the host (
Perform these steps inside the VM (
Running Whonix ™ in a Nested Virtual Machine
Only Whonix ™ 64-bit builds are available for download; see 32-bit or 64-bit? for reasons why. Some virtualizers provide nil or limited support for running nested VMs that require 64-bit. This might be an issue when trying to run Whonix ™ in a nested virtual machine.
- https://www.webopedia.com/TERM/N/nested-virtualization.html [archive]
Hardware-assisted Nested virtualization on Intel CPUs has been available starting with VirtualBox 6.1.0
Whonix-Workstation-XFCEwith the actual name of the VM, for example if the VM was renamed or multiple Whonix-Workstation ™ are in use. The following command works on Linux. It is untested on Windows but it should be possible to make this command work. Its purpose is adding VBoxManage to PATH (if that is not the default) or using the full path to VBoxManage.
VBoxManage modifyvm Whonix-Workstation-XFCE --nested-hw-virt on
- https://www.virtualbox.org/ticket/19500 [archive]
vboxmanage "Whonix-Workstation" modifyvm --ioapic on
So does enabling ACPI. Enabling ACPI in all VMs significantly speeds up the "inside" VM (
vboxmanage "Whonix-Workstation" modifyvm --acpi on
ACPI is the current industry standard to allow OSes to recognize hardware, configure motherboards and other devices and manage power. As most computers contain this feature and Windows and Linux support ACPI, it is also enabled by default in Oracle VM VirtualBox.
- These settings are in use for Whonix ™ VMs by default.