For the past year, I’ve been getting to know virtual machines (VMs). 

I’ve mostly been using VirtualBox on Windows 10 and Parallels on my Mac. A few weeks ago I thought it was time to also explore Windows Hyper-V. So, I turned it on. Big mistake – activating Hyper-V deactivated my ability to run 64-bit VMs within Virtual Box. Which was a problem, as I have an instructional set of model VMs set up on my classroom computer, which were then exported for use on the student computers, And I had about a week invested in creating these particular model VMs (installed software, settings, model networks. etc).

Now, I did the two things (activate Hyper-V) and then revisit my VirtualBox VMs about a week apart. So it took me a bit of time (and googling) to work back and determine what i needed to do. 
Although I went in and turned off the Hyper-V features within Apps and Features > Program and Features > Turn Windows Features On or Off, I also ran PowerShell commands to make sure it was good and off and wouldn’t come back on when I next updated my classroom computer system. 

The key thing I learned was that the reason the two virtualization applications don’t work well together is that Hyper-V is a Type I hypervisor for Windows 10 – and VirtualBox is a Type II hypervisor running within Windows 10.

References:
https://www.gilzow.com/blog/2017/03/12/you-cant-run-hyper-v-and-virtualbox-at-the-same-time/