How do I get my USB to work in windows xp guest. Ubuntu 16.04 host

You need to add your user account to the vboxusers group of your Ubuntu 16.04 installation.

To add your user to the vboxusers group, run the following command from a terminal:

sudo usermod -a -G vboxusers <username>

It is recommended after the command that you reboot your system, and after the reboot you should be able to add your USB devices to VirtualBox.

After you reboot, go to your main VirtualBox screen and select your VM and click Settings above:

Perform this step before you start any VMs. Once in your settings, choose USB and select USB 2.0 (EHCI) Controller, then on the right you can add the filter for your USB drives that you're going to use with the VM. Your USB device has to be plugged in so you can add your USB filter.

Use the second icon down on the right with the + on it to add your USB filter.

Before reboot:

After reboot:

Hope this helps!