Disk2VHD Tool for Physical to Virtual Hyper-V conversion

https://technet.microsoft.com/en-us/sysinternals/ee656415.aspx

Important restrictions

Note: Virtual PC supports a maximum virtual disk size of 127GB. If you create a VHD from a larger disk, even if you only include data from a smaller volume, it will not be accessible from a Virtual PC VM. 

In addition Virtual PC doesn't support the Multiprocessor Specification, which means that it won't be able to boot VHD's from multiprocessor Windows XP and Windows Server 2003 systems.

If you create a VHD from Windows XP or Windows Server 2003 and plan on booting the VHD within Virtual PC, select the "Prepare VHD for use in Virtual PC" option, which ensures that the HAL (Windows Hardware Abstraction Layer) installed in the VHD is compatible with Virtual PC.

Check this post out for a VHD vs VHDX comparison
https://www.netwatch.me/2015/03/19/vhd-vs-vhdx-hmm-whats-the-difference/