Hypervisor security protects the virtualization layer. VM escape = attacking hypervisor from a guest VM to access the host or other VMs. Type 1 (bare-metal) is more secure than Type 2 (hosted).
Type 1 hypervisors (VMware ESXi, Hyper-V, Xen) run directly on hardware. Type 2 (VirtualBox, VMware Workstation) run on an OS. VM escape is rare but catastrophic — patch hypervisors quickly.