I see virtualization inventory under virtual machine option. But guest machines doesn't have rich inventory. Is there any way to discover virtual/guest machines with standard data collection mode with agentless or agent method?

Discovery, Inventory and Compliance General
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While you collect virtualization basic details for host (ESXi, Hyper-V, XEN) along with basic details of all virtual/guest machines on-boarded on base host, we can always discover virtual/guest machines using agent-less standard protocols WMI, SSH, SNMP WBEM or with agents. In this way you get rich set of inventory.

For agentless discovery, depending on the OS type, configure credential profile such as WMI, SSH,etc and then configure those virtual/guest machine IP addresses as part of discovery rule.

For agent based discovery, you will have to roll out specific agents on virtual/guest machines based on operating system installed.