What is a difference between Azure Migrate: Server Migration agentless and agent-based migration?

Agentless migrations do not require any software (agents) to be deployed on the source VMs/servers being migrated. The agentless option orchestrates replication by integrating with the functionality provided by the virtualization provider. The Agentless replication options are available for VMware VMs and Hyper-V VMs.

Agent-based migrations require Azure Migrate software (agents) to be installed on the source VMs/machines to be migrated. The agent-based option doesn’t rely on the virtualization platform for the replication functionality. Therefore, it can be used with any server running an x86/x64 architecture and a version of an operating system supported by the agent-based replication method.

Agent-based migration option can be used for VMware VMs, Hyper-V VMs, physical servers, VMs running on AWS, VMs running on GCP, or VMs running on a different virtualization provider. The agent-based migration treats your machines as physical servers for migration.

While the agentless migration offers another convenience and simplicity over the agent-based replication options for the supported scenarios (VMware and Hyper-V), you may want to consider using the agent-based scenario for the following use cases:

IOPS constrained environment: Agentless replication uses snapshots and consumes storage IOPS/bandwidth. We recommend the agent-based migration method if there are constraints on storage/IOPS in your environment.

If you don't have a vCenter Server, you can treat your VMware VMs as physical servers and use the agent-based migration workflow.