[xwiki-users] Best Practice for multiple "fully functional" instances?

Hello! We are currently discussing the possibilities on how to configure our system for the following scenarios: We have multiple XWiki instances for several users. Since the users want to have full admin rights we cannot use the xwiki farm. For security reasons we are deploying each instance into a virtual machine (KVM) with its own Tomcat and Database. Using this kind of setup seems to be most secure and flexible in terms of miss configured tomcat installations, SQL-injections and file-system (on purpose or by mistake) access though scripting. The obvious down side is the huge overhead which comes with the virtualization. So I was wondering what kind of setups you are using. Can I deploy the application into some kind of tomcat-chroot-environment? What kind of database setup is possible? Is it possible to restrict access to a certain database on an application bases (much like you can restrict access based on the connecting ip-address). The Database has the be PostgreSQL. I appreciate all kind of inputs. Cheers, Stephanie.