Comment # 5 on bug 1123826 from Charles Arnold

Just to clarify what shipped with these distros you mentioned.

openSUSE Leap 42.3 is based on SLE12-SP3.
Current update channel Xen version for 42.3: xen-4.9.3_03-34.1
Current update channel Kernel version for 42.3: kernel-default-4.4.165-81.1

Kernel version 4.12 was introduced in SLE12-SP4 and SLE15. SLE12-SP4 has no
equivalent openSUSE Leap version. Xen 4.11 shipped on SLE12-SP4.

The next Leap version is openSUSE Leap 15.0 which is based on SLE15.
These two distros shipped with Xen version 4.10 and kernel version 4.12

Running kernel 4.12 on Leap 42.3 with Xen 4.9 is actually an untested
configuration.