developer is officially maintaining the package. The rationale for changing this from "tech-pkg" to "pkgsrc-users" is that it implies that any user can try to maintain the package (by submitting patches to the mailing list). Since the folks most likely to care about the package are the folks that want to use it or are already using it, this would leverage the energy of users who aren't developers.
Xen is a virtual machine monitor for x86 hardware (runs only on i686-class CPUs), which supports running multiple guests operating systems on a single machine. Guest OSes (also called "domains") require a modified kernel which supports Xen hypercalls in replacement to access to the physical hardware. At boot, the xen kernel is loaded (via grub) along with the guest kernel for the first domain (called domain0). domain0 has privileges to access the physical hardware (PCI and ISA devices), administrate other domains and provide virtual devices (disks and network) to other domains. This package contains the Xen kernel itself.