XCP-ng is Free Software.
It is built on top of various components including the Linux kernel (GPLv2), Xen (GPLv2, LGPLv2+ or BSD, depending on the components) and many more F/OSS components (the license for each is defined in the License tag of each RPM).
A file compiling the texts of the licenses is available at https://github.com/xcp-ng/branding-xcp-ng/blob/master/branding/LICENSES. For the licenses of a specific release of XCP-ng, use the select box on the top and select the tag that corresponds to the release you are interested it.
It is also present in the installer ISO image.
All features are available for free. Pro support is available from https://xcp-ng.com/
XCP-ng's documentation itself is published under the Creative Commons BY SA 2.0 license.