The PSR-E3xx models don't have a "DSP" function per se, but their Reverb and Chorus functions are DSP functions.
There are at least two reasons why keyboards often have separate Reverb and Chorus functions in addition to-- or in some cases instead of-- a more generic DSP function: (1) Reverb and Chorus are so commonly used that it's very handy to give them their own functions; and (2) by having separate functions for Reverb and Chorus, as opposed to just having a single DSP function, it's possible to use Reverb and Chorus at the same time, rather than having to choose between one or the other.
Some keyboards do have a more all-purpose DSP function in addition to separate Reverb and Chorus functions, for using a wide variety of DSP effects that don't fall within the Reverb and Chorus categories. This additional DSP function usually includes all effects that are available in the separate Reverb and Chorus functions, which lets you use two different types of Reverb at the same time, or two different types of Chorus, or just some third type of effect in addition to a Reverb and a Chorus effect. But I don't think any of the PSR-E3xx models have a general DSP function like that, just Reverb and Chorus functions.
One thing to be aware of with respect to Reverb, Chorus, and DSP is that they're usually system effects, meaning you can normally choose only one type of Reverb, one type of Chorus (if available on your keyboard), and one type of DSP effect (if available on your keyboard), which apply to the keyboard's entire sound system as a whole rather than being limited to just a specific part. In other words, you can choose just one Reverb Type, rather than being able to choose one Reverb Type for the Main Voice, a second Reverb Type for the Dual Voice, and a third Reverb Type for the Split Voice; and likewise with the Chorus Type.
However, once you choose the Reverb Type and Chorus Type that you want to use, you can typically choose how much of that Reverb effect, and how much of that Chorus effect, you want to apply to the Main Voice, how much you want to apply to the Dual Voice, and how much you want to apply to the Split Voice. This lets you use a heavy Reverb effect for the Main Voice, but turn off the Reverb effect entirely for the Dual Voice, and things like that.
The Function List in your keyboard's Owner's Manual says (on page 46) what system-wide and what voice-specific settings are available to you, and the Effect Type List says (on page 75) the different Reverb Types and Chorus Types you can choose from. In fact, I see that the PSR-EW300 doesn't let you set a different Reverb Level for the Main Voice, Dual Voice, and Split Voice-- you can set the overall Reverb Level for the system as a whole, but not for each of the three voices. On the other hand, you can set a different Chorus Depth for each of the three voices.
