how to backup vm?

[linbing]

freenas 11.3-u5

windows server 2019 vm

can output qcow2 file?

 

[sretalla]

can output qcow2 file?

Not exactly, but you can use dd to write the zvol to a raw file and then use tools to convert it.
dd if=/dev/zvol/<pool>/<volume> of=file.raw bs=1m
qemu-img convert -f raw -O qcow2 file.raw file.qcow2 on your kvm host