If you want to enjoy a full comfort of using the Ubuntu virtualized to a Hyper-V instance, do the following:

1. run sudo gedit /etc/default/grub in your Terminal in order to edit GRUB file
2. find the line which starts with GRUB_CMDLINE_LINUX_DEFAULT
3. add video=hyperv_fb:1920x1080 (or other resolution wanted) - in the end your file should look like as pictured below:
4. save the file and exit
5. run sudo update-grub
6. reboot the system (meaning the VM instance of Ubuntu, of course)