Update: It seems the upgrade process has overwritten the default locations where my virtual machines were stored. If you did not yet upgrade and care for your Hyper-V VMs, be very careful. Make sure you move the virtual machines to a safe location first.
After upgrading my Surface 3 Pro to Windows 10 I noticed that Hyper-V was no longer installed. I spend some time in the new Settings app but could not find a way to enable Hyper-V again.
Fortunately this issue is very easy to fix. Open Control Panel and navigate to Programs, Turn Windows features on or off. Select Hyper-V and click OK to continue.
But before you do this, please check the full list and select or deselect any other features you want to install or remove.