The Dom0 or control domain has traditionally been a monolithic privileged virtual machine. However, the original intention was for Dom0 to be carefully broken into several privileged service domains - termed Dom0 disaggregation. Qubes OS, OpenXT, Citrix XenClient, and Xoar have made the case for more disaggregation of Dom0 for purposes of better security, reliablity, isolation, and auditability.
"Although domain 0 disaggregation is not new it is seldom used in practise and much of its use is focussed on providing enhanced security. Citrix XenServer will be moving towards a disaggregated domain 0 in order to provide better security, scalability, performance, reliability, supportability and flexibility."
Project “Windsor”: Domain 0 Disaggregation for XenServerXCP: