A virtualization stack is the full set of layers that make virtual infrastructure work: the hypervisor, the management domain or tools, virtual devices, and the guest operating systems running inside each virtual machine. Together, these components control how compute, memory, storage, and network resources are shared on a host.
It matters in cyber security because each layer is part of the trust boundary. A flaw in the hypervisor can threaten isolation between guests, while weaknesses in management software or privileged control domains can expose the whole platform to takeover. Defenders treat the stack as one security unit: they inventory versions, apply vendor-specific patches, harden administrative access, and monitor for signs of escape attempts, unauthorized VM control, or abuse of privileged management paths.