Proxmox Virtual Environment's source code is published under the free software license GNU AGPL, v3 and thus is freely available for download, use and share. You can download the Proxmox VE ISO installer or inspect the code in the public code repository (git).

GNU AGPLv3: http://www.gnu.org/licenses/agpl-3.0.en.html

Public Code Repository: https://git.proxmox.com/

Web Site: https://www.proxmox.com/en/proxmox-ve