Maybe you installed Ubuntu / Xubuntu side by side with Windows 10, and later decided that after all you want to run Linux in Virtual Machine inside Windows. Then you need to remove your Xubuntu partition and Grub in order to boot directly to Windows.
Before doing anything you should make sure that you have an USB resque stick for Windows. That can be made by using the appropriate tool in Control Panel. And of course back up everything valuable, especially from Linux partition.
Using Disk Management you can delete the Linux partition and extend the Windows partition to the empty space. Then in UEFI you can put your Windows Boot loader on top of the boot order list, and delete the Linux boot option. Save and exit.
After that is up to you if you want to reinstall Windows to start from a clean table, or not. And then into installing Virtual Machine and Xubuntu in it.