Proxmox is an open source platform that can be used to manage enterprise virtualization. Proxmox takes advantage of the KVM hypervisor and Linux Containers (LXC). Yes, you can also run containers on Proxmox (we will not cover this as we will focus on it running VMs). Proxmox is based on Linux, so there is also a free community edition (CE) that is available to download. We will use the CE edition of Proxmox.
Simply, Proxmox is a type-1 hypervisor.
A simple definition of open source is when the source code is able to be viewed by everyone. By allowing anyone to view the source code it promotes collaboration and sharing of ideas.
To gain a better understanding of what is open source please read the following links. These links will provide you with a very great perspective on what open source is.
KVM (Kernel-based Virtual Machine) lets you turn Linux into a hypervisor. KVM is part of Linux and has been since the release of the 2.6.20 kernel. Since KVM is built into the Linux kernel it has all the features that you expect Linux to have. What KVM does is it turns Linux into a type-1 hypervisor.
Kernels are the most important part of an OS. Many people also say that the kernel is the OS, that everything else is just software/applications. This is an extremely important piece of the OS, so make sure you read about what a kernel is.