How to process current and previous IoT Hub event from an azure function?

Are these telemetry messages or property changes? Also what's the scale (number of devices)? To do this effectively you need to ensure you have both the current and previous values, which means storing the last reported value and timestamp externally as it could be a long time between. The Event Hub is not guaranteed to have all past messages (default is 24h), so if there's a long lag between messages it's not the right store to rely on.

Durable Entities can be used to store state (using something similar to the Actor Model). These are persisted in Azure Storage so at extremely high throughput a memory-only calculation option might make sense with delayed persistence, but you can build a memory-caching layer into your function to help if needed. This is likely going to be the best bet for what you want to do.

For most people the performance hit of going to Azure storage and back is minimal and Durable Entities will be the easiest path forward.

If you are doing it in a near-real time stream, the best solution is to use Azsure Streaming Analytics using the LAG operator. ASA has a bunch of useful features that you will need such as the PARTITION BY and event ordering policies. Beware, ASA can be expensive to run and hard to work with, but is a good service for commercial solutions.

If you don't need near-real time, a plain 'ol python script that queries (blob) persisted data is a good option, and can be wrapped up in an Azure function if it doesn't take too long to run.

Azure functions are not recommended for stateful message processing. You simply have insufficient control of the number of function instances running, the size of the batch, etc. So it is impossible to consistently and confidently know what the 'previous' timeseries value is. With Azure functions, you have to develop assuming that concurrency is never going to be an issue, which you cannot do with streaming IoT data.