[CentOS-virt] Differences between pc and q35

Hi all,

  What are the differences between pc and q35?? By default, 
virt-manager+libvirt setups kvm guest machine as a pc-i440fx-rhel7.1.0.

[hicheck at ckvm015 ~]$ /usr/libexec/qemu-kvm -machine ?
Supported machines are:
pc                   RHEL 7.1.0 PC (i440FX + PIIX, 1996) (alias of 
pc-i440fx-rhel7.1.0)
pc-i440fx-rhel7.1.0  RHEL 7.1.0 PC (i440FX + PIIX, 1996) (default)
pc-i440fx-rhel7.0.0  RHEL 7.0.0 PC (i440FX + PIIX, 1996)
rhel6.6.0            RHEL 6.6.0 PC
rhel6.5.0            RHEL 6.5.0 PC
rhel6.4.0            RHEL 6.4.0 PC
rhel6.3.0            RHEL 6.3.0 PC
rhel6.2.0            RHEL 6.2.0 PC
rhel6.1.0            RHEL 6.1.0 PC
rhel6.0.0            RHEL 6.0.0 PC
q35                  RHEL-7.1.0 PC (Q35 + ICH9, 2009) (alias of 
pc-q35-rhel7.1.0)
pc-q35-rhel7.1.0     RHEL-7.1.0 PC (Q35 + ICH9, 2009)
pc-q35-rhel7.0.0     RHEL-7.0.0 PC (Q35 + ICH9, 2009)
none                 empty machine

But according to this, q35 is more new virtual hardware. Why??