Exporting a Virtual Machine from Xenserver and then importing it into Hyper-V is not officially supported from any documentation I can find on-line. If you have need for this conversion, and your Virtual Disks are in VHD format, this is really quite simple. The below tutorial will walk you through the steps I took to move a VM from Xenserver to Hyper-V. I have not tested this with virtual disk formats other than the VHD format, so other scenarios are untested and unknown to work or not.
First step: In XenCenter, export the target VM (VM must be shutdown to export) with the OVF/OVA option and be sure to not choose the option to create a single OVA file. Importing to Hyper-V will not work with the single OVA file export. All the remaining default options are fine, choose to verify the export if you desire.
Second step: On the Hyper-V host, create a folder on the hard drive where you will want your VM's files to live. Once the folder is created, copy the VHD and OVF file from the first step to this folder (The VHD file will be named with a large string of letters and numbers).
Third step: In your Hyper-V manager, create a new Virtual Machine and name it as desired. For the VM destination, select the folder created in the second step that contains the VHD and OVF files. Assign the memory and network options and for the virtual disk, select the exported VHD from the above step.
Fourth step: Adjust and fine tune the remaining settings (virtual processors etc...) and then power it up, your all done!
I have tested this by exporting VM's from Xenserver 6.0.2 and 6.1 and importing them to Hyper-V 2012 running in full Server 2012 hosts.