Xen
A type 1 hypervisor out of the Cambridge University Computer Laboratory, published in 2003. It built the entire first generation of public cloud — AWS EC2 ran on Xen for over a decade.
The innovation was paravirtualization: instead of emulating hardware, the guest operating system was modified to call the hypervisor directly, which gave near-native performance before CPU vendors shipped virtualization extensions. The architecture splits the system into dom0, a privileged Linux domain owning the drivers, and the guests' domU. Once Intel VT-x and AMD-V arrived, paravirtualization mattered less and KVM — which sits inside the Linux kernel rather than beneath it — took over much of the usage. Amazon moved to its KVM-based Nitro platform from 2017. Xen lives on in XCP-ng, Citrix Hypervisor and Qubes OS.