Proxmox VE 2.x support Software RAID, but Proxmox VE 3.x up to 3.3 does not support Software RAID (mdraid). That means, its not tested in our labs and its not recommended, but its still used by experience users. If you want to run a supported configuration, go for hardware RAID.
NOTE: Proxmox VE since 3.4 support Software RAID if you choose ZFS storage.
If you still want to use software raid, you can create required raid level during Debian installation and then install Proxmox VE. Look at the external links below and the following pages:
- Install Proxmox VE on Debian Squeeze
- Install Proxmox VE on Debian Wheezy
- Install Proxmox VE on Debian Jessie