The software model is the oldest approach, which consists of a database to store all of the information, an interface to manipulate it, and a system to take that information and distribute it to currently deployed stand-alone DNS and DHCP servers. The software model manages other DNS and DHCP servers, but those servers are not part of this model and need to be purchased from another source. The advantage of this solution is that existing infrastructure does not need to be replaced, and access into the database is simpler for outside scripts. The pricing model is the total of the software cost, yearly maintenance and number of hosts.
The appliance model is a one-shop solution where the database, DNS and DHCP servers are all supplied as simple to manage appliances which are easily maintained. This solution is normally much easier to support with a higher ROI, but it involves a forklift upgrade (your current infrastructure is replaced with the new appliances). The pricing model is normally simpler; the cost of the appliance + yearly maintenance, and not including a per IP fee. The appliance model may also include virtual appliances.
The hybrid model uses appliances where there is less chance of having to maintain an existing platform. For example, most organizations do not currently have an IP Address Management server so there is no server to maintain or upgrade with new software. However, software can be installed on existing DNS or DHCP servers to allow them to be controlled by the management appliance. These software controlled servers could also be mixed with appliances.