The method of accessing the console of a VM you started manually with the
kvm command would depend on which arguments you used. By default, KVM will try to use SDL to display the VGA output. This limits output to only the current logged in user (X session, basically).
A better way to do it is to add
-vnc :0 to your arguments. This will tell KVM to redirect the VGA display over a VNC session on the standard VNC port of 5900 (for other ports, see below). You can then access the console with any VNC viewer; tell it to use hostname
localhost and port
To use a different VNC port, just do
-vnc :1 for port 5901,
-vnc :2 for port 5902, and so on...
virsh does all this for you behind the scenes. In fact
virt-viewer is simply a minimal VNC client.