It isn't true that KVM does not have Dom0. But it reside in different layer, put the fact that both KVM and Xen -- Dom0 is privileged and handle device drivers + hardware, weather is in top or bottom of virtualization later.
For Xen, Xen Hypervisor is a controller too grant access Real hardware/cpus, but Dom0 is virtually both HW driver and admin system.
For KVM, It is application + kernel part too grant access Real hardware/cpus (using processor assisted), but system running KVM is both HW driver and admin system.
In both cases, if parent or dom0 cracked, whole system is compromised.
I agree that if Xen code is too much impact on x86 core arch code it should not merge until this fixes, but again KVM is not Xen replacement at all. Even with xenner which is really KVM but Xen guest.
I can say that Xen lacks support from its users. If Xen shipped in kernel it would heavily tested. But now it's not, even Xen currently keep track latest kernel inside git, most people seems to use stable and old Xen kernel and not many are working on new Xen Dom0 kernel.
And again, KVM is not thing that would replace Xen and not replace each other. I cannot see any benefits for replace Xen with KVM for now (I running a couple servers using Xen, some of those not support VT-d or AMD-V).
Topics of this problem is that Xen cause too much changes of the core x86 code and seems not to clean enough.