Does anyone know, wheter the Caravel Harness supports diff signaling for e.g. PCIe or HDMI? Or due I need to use something like a high speed differential line driver?

As far as I know (please correct me if I am wrong), Caravel does not provide differential IO Pads yet, but it provides Analog IO Pads which likely could be used with your own differential logic attached to them. For driving PCIe or HDMI, you also need a compatible SerDes, I heard about several projects trying to implement that but I haven't seen working results yet.

you're correct Philipp

Differential drivers are high on my wish list. . .