i installed elastix iso that consists of centos 5.6 in mikrotik's kvm. and it is successfully running. but i was confused if it support dahdi hardware. since elastix needs dahdi hardware to connect to pstn lines.
I believe you're referring to a physical PCI card for connecting to PSTN lines yes?
If this is the case then you're not going to have much luck using KVM as there's no-way in the mikrotik implementation to configure hardware pass through to the virtual machines. It might be a better idea for you to run ESXi on the physical box then run both RouterOS and Elastix as vm's of the host machine.
==> you're not going to have much luck using KVM as there's no-way in the mikrotik implementation to configure hardware pass through to the virtual machines
Because the mikrotik KVM implementation is limited on features; only what can be easily provided access via winbox/terminal is offered.
The only real virtual hardware option offered is the ability to create the virtual ethernet interfaces required for a machine to be able to connect out to the internet.
You would not have an easy way of connecting POTS lines without some sort of gateway device. But you could easily purchase sip trunks from a vendor such as phonebooth.com (no hardware needed). If you must have POTS line integration, you can get a VOIP gateway such as this: http://www.voipsupply.com/audiocodes-mp-118-fxo . But they are expensive and troublesome to configure. Although 8-port digium FXO cards are also pretty expensive.