I've been using KVM with great results on Slackware but one thing about Xen 4 that appeals to me is VGA and PCI passthru. I did try to install the Xen Slackbuild some time ago but failed. I'm afraid I can't remember why now. As far as I remember the Slackbuild was just out a day or so, so perhaps there were unresolved issues in the early days.
How is Xen now? Does it run as well as KVM, for HVM as well as PV guests? Does virt-manager allow you to create Xen VMs or just KVM VMs? I'm very happy with KVM on Slackware (and Xen on NetBSD) but as I say, the Xen 4 release has some enticing features which pretty much do away with dual- and multi-booting. Full-throttle graphics in a VM? I'll have some of that please!

In theory, Xen should run fine on Slackware64.

And yes (by design!) virt-manager is agnostic to either KVM or Xen.

However, I'm reading (the Most Excellent!) "Book of Xen", by Chris Takemura and Luke S. Crawford. I get the impression that, although you *should* be able to do it OK, there might be some "interesting challenges" with Slackware (as opposed to, say Centos or Debian).

I bought the latest edition of that book and indeed it is very good.
Do you mean there are interesting challenges installing Xen 4 on Slackware or just the virt-manager?