You can use both QVD and QVW files in both version 9 or 10 regardless the version they were created with.
Anyway, there's always a section in the Release Notes of every new version of QlikView about Cross-Release compatibility.
Pasted from latest SR2 release notes:
Cross-release compatibilityThere are no problems using QlikView Desktop 10 together with QlikView Desktop 9 inparallel on the same computer. However, the QlikView 10 Plug-in for Microsoft InternetExplorer will conflict with previously installed versions of QlikView Plug-in. Either the previousversion needs to be uninstalled, or the QlikView OCX feature from the QlikView 10installation needs to be de-selected.QlikView has a common file format for versions 6, 7, 8, 9 and 10.