We are in the process to ditch our physical servers and move to virtualization. I have tested a few solutions, and we have decided to use Xen - still open between "Linux Xen" or Citrix Xenserver.
We do want HA, even though our servers & applications running are not strictly business critical - downtimes will mainly affect our developers (which of course comes with a cost & annoyance, but no impact for clients). Maybe "automatic" failover is not absolutely necessary, but we must be able to manually shut down one host, one storage server without affecting running guests.
There will be 8-12 guests running (only Linux servers w/o GUI), total storage used today for all is around 100GB.

So, the plan is to have 2 nodes in one pool, my main concern is configuring storage. I'm thinking a Storage repository with 2 physical servers running DRBD here. Any objections?

Looking at the minimal configuration:
2 physical servers as Xen hosts. One 1TB disk each which is handled through DRBD and used as a Storage repository.
With this scenario, we should be able to move all running guests to Xen-host-2, switch DRBD master to Xen-host-2 and then shutdown Xen-host-1 without affecting running guests - is this correct?
And if one Xen-host goes down unplanned, we will be able to get all guests up & running within, say, 15 minutes?
Do you think this is a decent solution for our small company, or should we go for separate storage?
Can this be done with Citrix Xenserver?
Any other advice is appreciated!

if you use DRBD, you have to go with active/passive mode. That means one of the hosts will be useless for running VMs.
I would go with oVirt and a central storage unit (that can be made redundant with DRBD now or at a later stage), you get HA out of the box, no need to play with corosync/pacemaker/etc, basically a complete, vsphere-like solution at the price of zero.

But it's just one drive that will be handled by DRBD, not the system drive.
Do you mean that it can't be done that way, that DRBD is "all or nothing? That's not how I have understood it?

And we have decided to use Xen since it is far more used, a lot easier to get professional help should we need that. I did try oVirt in our early stages though, I wasn't very happy with it. The web interface was extremely slow, when rebooting disks were not unmounted properly so the system hang. Had to physically shut down the server.

DRBD has two modes - active/active and active/passive. The first mode is a very bad idea in production, especially for VMs. You will end up with disk corruption, data loss and splitbrain.
With a/p mode, you can only read/write to one of the disks in the pair, the other has to be in standby.

oVirt has come a very log way, and support for it mainly happens in the users@ovirt.org list. That list is amazingly responsive and I haven't seen anyone leave the list without getting an answer, usually directly from developers.

As for xen vs kvm, you don't want to go there. Xen is still used in amazon, but I doubt you match their size and scale. Moreover, their use of xen is for legacy reasons and it is too late to change. KVM is used in 80+ percent of openstack deployments. It has commercial support from red hat, ibm and lots of other vendors. In short, you might want to check your sources.