![[attachments/Screenshot 2023-05-26 at 4.54.48 PM.png]] For the **hosted VMM** we have a virtual machine (like Virtualbox) that lets us run Windows on a Linux operating system. For the **hostless VMM** we have the operating systems **on top of** the VMM. ![[attachments/Screenshot 2023-05-26 at 4.58.47 PM.png]] If there is no VMM then system calls are sent from a process to the operating system. If there is a VMM then the system call first gets sent to the VMM (has the highest privileges) and then is sent to the guest OS (lower privileges than VMM). [[TCB - Trusted Computing Base|TCB]] in Hostless virtualization - VMM is the TCB - Smaller, more likely to be correct - Partitions hardware resources among different virtual machines (VMs) - Guest OS in VM manages resources Popular VMMs or hypervisors - Xen - VMWare ESX - Hyper V - KVM