Although Xen is probably an unlikely candidate as guest OS don't run nicely OOTB from what I know. I too looked at Xen initially when building a headless VM server but discounted it for a range of reasons (complexity being one of them).

It does seems likely that a Headless VirtualBox Server ISO will make it into the part 2 release of TKL v11.x (although that has been significantly delayed a number of times so not sure when that'll happen - soon hopefully). In the meantime prior to the official appliance being created you could apply the patch provided by Rik (see the forum thread here) or download his precreated ISO here. VBox may not really be what you are after though?

Otherwise it is likely that a TKL 'Meta-appliance' may be produced at some point which will support installation of numerous TKL guest appliances onto a single bare metal machine. However it is likely that this setup will make use of LXC (or OVZ) container style virtualisation which only supports Linux OS (so again may not suit your needs).

Failing those possibilities I can highly recommend ProxmoxVE. It is a commercial grade (free and) open source hypervisor OS based on Debian which installs from CD (downloadable ISO). It has an easy to use WebUI and supports 2 types of virtualisation: OVZ (container style - Linux only) and KVM (full virtualisation - any OS). It also supports clustering OOTB so you can have multiple hardware nodes running and migrate live VMs between the hardware nodes, as well as live backups.

I use PVE both at home (for my personal fileserver and other TKL appliances as well as development) and at work (I have virtualised all our server infrastructure under PVE). There are others here that have tried PVE and I'm yet to hear any serious complaints.

I recommend Proxmox VE as well. I run it and it is absolutely fantastic.