For openSUSE Leap 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:smarty12:virtualisation/openSUSE_Leap_15.6/home:smarty12:virtualisation.repo
zypper refresh
zypper install xen
For openSUSE Leap 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:smarty12:virtualisation/openSUSE_Leap_15.4/home:smarty12:virtualisation.repo
zypper refresh
zypper install xen
For openSUSE Leap 15.3 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:smarty12:virtualisation/openSUSE_Leap_15.3/home:smarty12:virtualisation.repo
zypper refresh
zypper install xen