Use a delegate if you want to talk to only one object. For example, a tableView has a delegate – only one object should be responsible for dealing with it.
Use notifications if you want to tell everyone that something has happened. For example in low memory situations a notification is sent telling your app that there has been a memory warning. Because lots of objects in your app might want to lower their memory usage it’s a notification.
I don’t think KVO is a good idea at all and try not to use it but, if you want to find out if a property has changed you can listen for changes.
Hope that helps.