They differ somewhat in what data is available, but more importantly how that data is obtained.

ExpertFlyer is web based, EF pays to access and publish the availability displayed. Very reliable and no features that I know of have been removed or unavailable at any point.

KVS is a windows application which scrapes data from various websites, sometimes using login information you provide to perform award searches (which you could do yourself for free on those websites). You are not actually paying for the data, but rather for the tool which needs constant updates so that it can scrape the data from websites as they're updated. The options available can change or disappear completely if the website owners change/remove the ability for KVS to scrape their data (often illegally against their website T&C's).