Proxmox VE uses a Linux kernel and is based on the Debian GNU/Linux Distribution. The source code of Proxmox VE is released under the GNU Affero General Public License, version 3. This means that you are free to inspect the source code at any time or contribute to the project yourself.
At Proxmox we are committed to use open source software whenever possible. Using open source software guarantees full access to all functionalities - as well as high security and reliability. We think that everybody should have the right to access the source code of a software to run it, build on it, or submit changes back to the project. Everybody is encouraged to contribute while Proxmox ensures the product always meets professional quality criteria.
Open source software also helps to keep your costs low and makes your core infrastructure independent from a single vendor.