Virtualization changes everything. IOU(IOS on Unix) was a well-known secret on Sun Sparc platform. And then Cisco 7200/3600 on dynamics. Then IOL(IOS on Linux) with various front ends including famous GNS3 and EVE-NG. Then Cisco's official network simulator VIRL.
More and more manufacturers offer virtual appliances of their network products. Virtual appliances are offered in various formats to support VMware, KVM, MS Hypervisor, etc. Two of the popular formats are ova (for VMware) and qcow2 (for KVM).
It's pretty straight forward if you plan to run the virtual appliance on physical host. For example, if you have VMware Workstation/ESXi you'll choose ova. If you have KVM, you'll choose qcow2. However, it's not so straight forward if you plan to use them with GNS3.
GNS3, along the time has evolved a lot. GNS3 is a client/server application suite. The best practice is to use GNS3 VM, which is a Linux virtual machine with GNS3 server preinstalled. With GNS3 VM, you may have all kinds of software images (IOS, IOS-XR, NX-OS, Viptela, etc.) contained in a VM. Makes it easier to package, distribute and share your labs.
If you already have VMware (either Workstation or ESXi), you probably want the GSN3 VM run on VMware. How about those Cisco appliances? Shall you download the OVA file so that you may run them on VMware as well? Not exactly. In order to take advantage of GNS3's user friendly GUI front end, You'll have to run Cisco appliances inside GNS3 VM. You'll have to use qcow2 (KVM format).
In theory, you COULD use OVA format. If you do so, you'll run Cisco appliances outside of the GNS3 VM. You could still connect those appliances to your GNS3 virtual routers. But it would be very cumbersome.
In short, if you plan to use GNS3, always download the qcow2 format.
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