Found out the hard way on this that yes you can install snmpd on the xen domu machines but, you have a very limited amount of snmp mibs to play with, things you can not do unless you create your own snmp mib file.
- Disk space
- CPU
- Connected users to machine
If you do some googling though you will find that some people have written some perl scripts which you can use in conjunction with snmp to collect this information.