(no major updates with WS2019)

Hyper-V/Hyper-V Server supports Differencing Disks with Server 2012 / R2 and 2008 R2. Differencing disks allow for the quick and space-saving creation of virtual machines by pointing to a shared parent disk and storing unique changes (writes) to the individual machines in a difference file that remains linked to the parent image. Reads will be served from the parent (unless the block has already been changed and writes accumulate in the difference file - increasing storage requirements over time).
With Server 2008 R2 Microsoft recommends though to avoid using differencing disks on virtual machines that run server workloads in a production environment (as running out of disk space when using diff disks can cause virtual machines to pause unexpectedly), see http://bit.ly/UxL3DK

Also with Server 2012 / R2, difference disks are typically used for virtual machines with shorter life cycle (i.e. lab environments or pooled VDI environments) due to the linked nature of the images (i.e. updating the parent breaks the link). For Server 2012 MS has not (yet?) updated the above article and only makes general performance recommendation in this document: http://bit.ly/UxHfT9
Quote: Difference disk Having only a few snapshots can elevate the CPU usage of storage I/So, but might not noticeably affect performance except in highly I/O-intensive server workloads. However, having a large chain of snapshots can noticeably affect performance because reading from the VHD can require checking for the requested blocks in many differencing VHDs. Keeping snapshot chains short is important for maintaining good disk I/O performance.