LXC(Linux Containers)
Lightweight system containers that behave like a small VM but share the host kernel.
Proxmox makes heavy use of LXC for services that don't need a full VM. They start fast and use fewer resources than KVM virtual machines.
Related articles
KVM(Kernel-based Virtual Machine)
The Linux virtualization technology that runs full virtual machines with near-native performance.
Container
An isolated, self-contained package that bundles an application with everything it needs to run.
Hypervisor
Software that creates and manages virtual machines by allocating physical resources between them.