By default AVD is saving video and temporary files to the system drive which usually drive C:\ of your PC.

You can change the paths to downloads and temporary files folders to another drive like it described at AVD Settings page in the Downloads section.

To clean all AVD cache find Temporary files folder row at the AVD Settings page in the Downloads section, click on Open link and delete all content in the appeared folder.

You can also clear all temporary files using Disk Cleanup app or manually delete all content in Temp folder by next path: %SystemDrive%\Users\%username%\AppData\Local\Temp\AVD-XX