What is the difference between alarmones and riboswitches
First time I actually hear the word alarmone so bear with me okay?

"An Alarmone is an intracellular signal molecule that is produced due to harsh environmental factors. They regulate the gene expression at transcription level" &lt;- this probably refers to activating/inhibiting transcription factors and stuff like that. Looking at bit further into the wikipedia article from whence you copied the description, we see that ppGpp is considered an alarmone. 

According to the wikipedia article on ppGpp (https://en.wikipedia.org/wiki/Guanosine_pentaphosphate) we see e.g. this line "When (p)ppGpp associates with the promoter it affects the RNA polymerase enzyme's ability to bind and initiate transcription", so this alarmone can act by inhibiting RNA polymerase's ability to bind to the promoter.

A riboswitch on the other hand is a structural element INSIDE an RNA molecule that causes a conformational change upon binding a ligand. This conformational change then has an effect on transcription or translation etc. Thus, an alarmone could probably bind to a riboswitch to exert its effect, but a riboswitch is a quite specific thing. 
You can read about some of the mechanisms in the wikipedia article https://en.wikipedia.org/wiki/Riboswitch

So basically, think of alarmones as signalling molecules and riboswitches as structural elements in transcripts that regulate gene expression upon ligand binding.