Well Linux has LXC which is supposed to be equivalent to jails (also see docker). But use whatever suits you.
Idk what's current for breaking out of VMs is. It might be good to pay attention to who is using the most entropy and make sure you don't run out. Most VMs use processor VT to isolate things (I don't think any 'jail' does this).
I think most providers use OpenStack (a suite of technologies). YMMV
Kind regardsWhat is the best (and if possible simplest) way to deal with this?I have been thinking about running Debian inside FreeBSD Jails or "The Warden". I have also been thinking about using Xen and installing several Debians on Debian.The solution can't be too complecated to maintain and I would prefer each user being completely seperated from the main OS and from other users.What does the big hosters do? What do they use?Due to this experience I would like to know what the best way to limit such problems is, especially when hosting web servers for users who may or may not installed unsecure applications on the web server.HiI have recently experienced a server being "hacked" due to a security problem with a PHP application that made it possible for the "hacker" to gain a web shell.