Architecturally, IoTivity had four fundametal units: Discovery, Data transmission, Data Management and Device Management. Similarly, AllJoyn focused on the following aspects: App Code, Service Frameworks Libraries, Core Library and either a Router or a Thin variant without the Router.
They also followed different protocols with IoTivity using CoAP and a MQTT plugin, whereas AllJoyn used its own bus based on D-Bus Wire protocol with its Gateway Agent providing connectors for REST, MQTT, XMPP and TR-069.
The array of supported platforms on these frameworks also varried with IoTivity extending its supports for Android, Arduino, darwin/iOS, Linux, Tizen, Windows and Yocto, while AllJoyn supporting Android, Arduino, iOS, Linux, OS X and Windows
Although, IoTivity and AllJoyn are now being merged under the Open Connectivity Forum(OCF) to act as the backbone to all things IoT. But, you can still check out IoTivity's merger-bridge project available on GitHub for their interoperability: https://github.com/iotivity/iotivity-alljoyn-bridge