1- virtualized domain controller need almost same policy as phiyiscal servers. inclued hardware resourse , backup policy ect
2- read Micorsoft recommed practice article like Considerations when hosting Active Directory domain controller in virtual hosting environments
3- it is better not to virtualized PDC. as pdc is the most important role in a domain. if you virtualized PDC do configure that virtual machine not sync time from host machine in settings.
4- do not use snapshot as backup or failover mathod! snapshot will make you crazy after restore the virtualized domain controller.
5- please use at least one Domain Controller in physical to avoid virual infrastructure failed.