Basically QVO is a QlikViewOutput files and they are used in exports when ever you export any chart info it will export like FileName.qvo and when you want to change its name rename it in any format like TXT, XLS, XLS, CSV it converted into that format and used as to read data from there.
And QVD is the Qlikview Data Base files they are used in qlikview and to compress the data is a QVD.
we can easily undderstand that if some file is in .qvo or any other format then we can convert by changing the extention.
whenver you export a chart and dont give any extention with name of file.then it will automatically stored into
where as qvd is nothing but qlikview document which native datasource for qlikview.