Is the xen version thats run by qubes itself modified or are the additional features like seamless handled by extra addons / separat applications?
Is it theoretically possible to install the modified xen version used by qubes standalone on another distro? (I am fully aware that this is not going to be anywhere near as secure)
Asking bc i am trying to get the seamless / unity mode that allows to display windows / applications from inside the vm directly on the host working on a project where it is unfortunately not practical / possible to run full qubes.
Thanks for any help & Happy New Year 
Both statements are true. For the additional patches to Xen used by Qubes, you can see GitHub - QubesOS/qubes-vmm-xen: Qubes component: vmm-xen. However most of them are backported patches or patches waiting to be merged upstream.
But IIUC seamless windows integration doesn’t need to modify Xen. Qubes GUI stack was implemented to achieve this. More info can be found here: GUI virtualization | Qubes OS
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