Xserve to our Mac Pro desktop

Any DisplayPort ports will do nothing for you here, as those are output ports. Not input.

FireWire and USB are I/O buses, and are not supported for console video output on Xserve.

While there are dedicated DisplayPort ports around on some Mac systems, DisplayPort is also a subset of what is available via Thunderbolt, and current Mac Pro does offer the superset; Thunderbolt and DisplayPort via Thunderbolt.

It might be (will be?) possible to locate dedicated video capture hardware that connects into the Mac Pro via Thunderbolt, and the software to process and display that. That won't be cheap, and you'll still have to get from DVI to whatever that external video capture adapter device accepts as input, if the external video capture adapter device doesn't already support DVI capture.

More typical here would be a connection from the Xserve directly to the display device. In this case, from DVI to the DisplayPort input on the LED Cinema Pro monitor; a DVI to Mini DisplayPort (MDP) adapter. There are adapters available that purport to convert from DVI to MDP; here is one. This adapter unfortunately doesn't particularly help you, as there's only one MDP input on the LED Cinema Pro monitor. You'd have to swap cables, unless you add a KVM switch — that's a switch-box that allows you to select which Keyboard, Video and Mouse is providing input into the monitor — with MDP connections or a KVM with DVI and MDP connections, and I've not looked for any of those. Another alternative is an additional LCD display with multiple inputs including DVI; an LCD that effectively has an integrated KVM embedded within it.

If not a monitor with multiple inputs, what most folks here end up using is screen sharing from Mac Pro into the Xserve, or acquiring SNMP-based remote monitoring software and using that. (No, I haven't looked for any of that SNMP software, but I'd expect it is available.) Or a KVM and a different monitor. This as the graphical monitoring tools that were available for Xserve are no longer supported on and don't work on current OS X releases.

Could you provide some additional background on the requirement or the general problem that you're working to resolve here? If I'm interpreting the question correctly — and I'm quite probably not — on why you're trying to connect the output of an Intel Xserve DVI to a Mac Pro DVI? Are you looking for some sort of DVI video capture? Or are you looking to share a monitor between the Xserve box and the Mac Pro? If so, which monitor are you using, and what ports does it offer? If there are no open ports, then an add-on VGA keyboard and video switch might work for your needs.