On a network level which is problably the best part to start:
http://www.linux-kvm.org/page/NetworkingBasically the host will work as a network bridge for its guests.
Apart from that internally the drivers are the only exposure to the hosts.
Think ; hdd, network,mem etc.
Before diving in way too deep since you seem to be new to virtualization I suggest you just read up on KVM/XEN/Hyper-V and go from there.
Basically the guest is sandboxed and has no connection to the host and is not aware it is a virtual machine.