Is one of those types of KVM switches better than the other in any way?
One faster than the other?
Is the switching instantaneous?
Transparency for the OS: Does the OS even know about the switch? Does it know when the switching is performed?
Is it just like plugging and unplugging the devices? (if it is, then switching PS/2 is clearly faster than USB) Or it's a more elaborated signal handling?
Is one of those types of KVM switches better than the other in any way?
Depends what you want to do, but for the most part they are equal. I often use a USB adapter on my PS/2 KVM switches with no problem. In most modern computers that still have PS/2 ports, the PS/2 bus is actually part of the USB bus. ---EDIT--- After doing some more research I found that the HIDs themselves are actually responsible for making the connection via USB or PS/2, not the motherboard. Please read comments.
One faster than the other?
No.
Is the switching instantaneous?
For the most part. The speed of switching has more to do with how fast your monitor reacts than anything else.
Transparency for the OS: Does the OS even know about the switch?
No.
Does it know when the switching is performed?
It may know that 'something' happened because several events will have taken place (i.e. you might have pressed keys on your keyboard to make the switch happen, the monitor connection might have been lost due to the switching, ect.), but it does not know that the events were caused by the KVM switch.
Is it just like plugging and unplugging the devices?
Yes and no. The result is just like unplugging and plugging in the devices, but without the side effects (for instance your computer will not have to recognize your USB keyboard again when you use a KVM switch, but it probably will if you unplug your keyboard then plug it back in). Hotswapping PS/2 connectors is not recommended because it can cause I/O controller damage.
(if it is, then switching PS/2 is clearly faster than USB) Or it's a more elaborated signal handling?
It is more than signal handling, it is magic.
---EDIT---
The first picture below is my primary KVM switch. I have a wireless keyboard and mouse combo attached to it. The wireless adapter has both a USB (for keyboard) and PS/2 connector. You can see that I use a passive USB to PS/2 adapter for the keyboard and then I plug the mouse in normally. If you had both USB keyboard AND mouse then you would just use two passive adapters instead of one. From here for normal purposes (only one KVM switch) you would just plug the leads of the KVM switch into your machines and go.
With that said, my particular setup is a bit more elaborate. I have three monitors and three computers that I access all the time (two servers and my workstation). I have it set up so that normally all three monitors are connected to my workstation, but I can use the far left monitor for server one when needed and the far right monitor for server two when needed. I can view all three computers at once with this setup since the first KVM switch controls the leftmost monitor, the second KVM switch controls the rightmost monitor, and the center monitor is dedicated to the workstation via HDMI.
The second picture shows how I accomplish this with a second KVM switch. The first lead of the second KVM switch is attached to a dual PS/2 to USB adapter (because I have no more PS/2 ports) which is attached to my workstation. The first lead of my first KVM switch is also attached to my workstation. The second lead of my second KVM switch is attached to my second server and the second lead of my first KVM switch is attached to my first server. My second KVM switch does not have a dedicated keyboard, instead I use the fourth lead of my first KVM switch as the input of my second KVM switch (no adapters required because the leads of the first KVM switch are PS/2 as are the inputs to the second KVM switch). To avoid any device changing conflicts I had to change the escape sequence of my second KVM switch (from CTRL -> SHIFT -> ALT -> DEVICE_NUMBER to something different (I used ALT -> SHIFT -> CTRL -> DEVICE_NUMBER)).
Please let me know if any of this is unclear.
FIRST PICTURE
SECOND PICTURE
After investigating the subject, it turns out that most USB KVM switches are just electrical contact switches, which means when you press the button, the USB devices are electrically disconnected from one PC and connected to the other, and so there is a big delay in the switching due to Windows detecting the devices.
If you google for "USB kvm delay" you are going to find a lot of complaints saying that USB switches have long delays. In the following pages search for the word "delay":
https://superuser.com/questions/2441
http://www.dslreports.com/forum/r20334032-Cheap-USB-KVM-as-USB-Switch
http://www.everythingusb.com/forums/showthread.php?s=834118c3975ac153fdca286cdb93434c&threadid=12043
http://marc-abramowitz.com/archives/2006/06/29/belkin-f1dk102u-usb-kvm-switch-a-piece-of-crap/
Ideally, a USB switch should present itself as a mouse and keyboard device to the PCs it is plugged into (which is what a PS/2 switch does). However, that is hardly the case for most USB switches out there, which the only thing they do is to electrically switch the devices from one PC to the other.
I found the PS2 quicker. We have one Aten KVM switch for PS2 and we switch between windows and linux machine, I have also second one - newer, also Aten (almost the same price level), but with usb. While the PS2 switch switches promptly, the usb one needs some time for the usb mouse and keyboard to be found (I can hear the typical connecting sound).
There should be no difference between switching time as far as USB or PS2 goes. I only use enterprise level KVM Switches, as these are more durable and turns out to be cheaper in the longer run.
