search in online help qvd, this is just an extract
Create QVD Files
A QVD file can be created by one of three different methods:
Explicit creation and naming using the Store command in the QlikView script. Simply state in the script that a previously read table or part thereof is to be exported to an explicitly named file at a location of your choice.
Automatic creation and maintenance from script. By preceding a load or select statement with the Buffer prefix, QlikView will automatically create a QVD file which under certain conditions can be used instead of the original data source when reloading data.
Explicit, manual creation and naming from layout or via Internal Macro Interpreter. Data can be exported from the QlikView layout via GUI commands or Automation macros. In the GUI you will find QVD as one of the possible export formats under the Export... command, found on the object menu of most sheet objects.
There is no difference between the resulting QVD files, e.g. with regard to reading speed etc.